Improve binary caching documentation, make more salient (#875)

People constantly ask about this, clearly without having found the
documentation that we do have about this. So we need to make it more
obvious:
- Simplify the setup documentation to emphasize the Hydra cache (which
is the only working one currently).
- Move the cache troubleshooting section to the top to make it more
obvious.
- Highlight a link to the troubleshooting section of the docs in the
README.
- Emphasize that there is more documentation than is linked in the
README.
- Add a section to the README that explicitly mentions cache issues, and
links to the corresponding sections of the docs.

Author: michaelpj

README.md

@@ -10,6 +10,21 @@ its dependencies into Nix code.
 
 - [Introduction](https://input-output-hk.github.io/haskell.nix/)
 - [Getting Started](https://input-output-hk.github.io/haskell.nix/tutorials/getting-started/)
+- [Troubleshooting](https://input-output-hk.github.io/haskell.nix/troubleshooting/)
+- Explore the documentation from there to find further topics.
+
+## Help! Something isn't working
+
+The #1 problem that people have when using `haskell.nix` is that they find themselves building GHC.
+This should not happen, but you *must* follow the `haskell.nix` setup instructions properly to avoid it.
+If you find this happening to you, please check that you have followed the 
+[getting started instructions](https://input-output-hk.github.io/haskell.nix/tutorials/getting-started/#setting-up-the-binary-cache) and
+consult the corresponding [troubleshooting section](https://input-output-hk.github.io/haskell.nix/troubleshooting/#why-am-i-building-ghc).
+
+The troubleshooting documentation also contains some help for other common issues.
+Please give it a read before asking on IRC or opening an issue.
+
+If you're still stuck, join the [#haskell.nix](https://www.irccloud.com/invite?channel=%23haskell.nix&hostname=irc.freenode.net&port=6697&ssl=1) channel on [irc.freenode.net](https://freenode.net/), or open an issue.
 
 ## Related repos
 
@@ -22,7 +37,3 @@ Haskell packages in Nix. It depends on other repos, which are:
 
 - [`stackage.nix`](https://github.com/input-output-hk/stackage.nix) — all of the [Stackage](https://www.stackage.org/) snapshots, converted to Nix expressions.
 
-## IRC Channel
-
-Join the [#haskell.nix](https://www.irccloud.com/invite?channel=%23haskell.nix&hostname=irc.freenode.net&port=6697&ssl=1) channel on [irc.freenode.net](https://freenode.net/) to get help or discuss
-the development of `haskell.nix` and `nix-tools`.

docs/reference/supported-ghc-versions.md

@@ -1,7 +1,7 @@
 # Supported GHC Versions
 
 The following versions of GHC built on the CI servers and should be included
-in the cache (for the default haskell.nix `nixpkgs`).
+in the binary cache (for the default haskell.nix `nixpkgs`).
 
 * 8.6.5 `compiler-nix-name = "ghc865";`
 * 8.8.3 `compiler-nix-name = "ghc883";`
@@ -27,6 +27,4 @@ The following GHC versions are not included in CI and will not be cached:
 See [overlays/bootstrap.nix](https://github.com/input-output-hk/haskell.nix/blob/master/overlays/bootstrap.nix)
 for a list of all the valid `compiler-nix-names`.
 
-See also:
-
-* [Instructions on adding new GHC versions](../dev/adding-new-ghc.md).
+See also: [Instructions on adding new GHC versions](../dev/adding-new-ghc.md).

docs/troubleshooting.md

@@ -1,5 +1,29 @@
 # Troubleshooting
 
+## Issues with building and garbage-collection
+
+### Why am I building GHC?
+
+It's easier to list the reverse: when will you *not* build GHC?
+
+- You have configured the [binary cache](tutorials/getting-started.md) correctly.
+- You are using one of the GHC versions which [we support](reference/supported-ghc-versions.md).
+- You are using one of the nixpkgs versions used by our CI (you can access the sources for these [through haskell.nix](tutorials/getting-started.md).
+
+If you think you are doing all of these and you still find you're building GHC, drop us a line.
+
+### Why am I building lots of Haskell packages?
+
+We don't generally cache much of Hackage (there's a lot of it!), except for the parts which are used by our tests.
+So this is expected, unfortunately.
+
+### How do I prevent the evaluation-time dependencies of my project from being garbage-collected?
+
+The `haskell-nix.roots "ghc884"` should include all the evaluation-time dependencies
+and the main build time dependencies of a project using ghc 8.8.4.
+So you can add that to the relevant GC root.
+In practice, if you're using a CI system like Hydra/Hercules, this means adding it to a job in `release.nix`/`ci.nix`.
+
 ## General troubleshooting when using `cabalProject`/`stackProject`/`project`
 
 ### Does the cabal/stack build work?
@@ -46,27 +70,3 @@ Where possible, try to do the configuration in your cabal/stack configuration, e
 This will ensure that the two builds agree.
 
 If you want or need to set some of them in Nix, try bringing the two into sync temporarily for troubleshooting.
-
-## Specific issues
-
-### Why am I building GHC?
-
-It's easier to list the reverse: when will you *not* build GHC?
-
-- You have configured the [binary caches](tutorials/getting-started.md) correctly.
-- You are using one of the GHC versions built by our CI (have a look in [ci.nix](../ci.nix) to see what gets built).
-- You are using one of the nixpkgs versions used by our CI (you can access the sources for these [through haskell.nix](tutorials/getting-started.md).
-
-If you think you are doing all of these and you still find you're building GHC, drop us a line.
-
-### Why am I building lots of Haskell packages?
-
-We don't generally cache much of Hackage (there's a lot of it!), except for the parts which are used by our tests.
-So this is expected, unfortunately.
-
-### How do I prevent the evaluation-time dependencies of my project from being garbage-collected?
-
-The `haskell-nix.roots "ghc884"` should include all the evaluation-time dependencies
-and the main build time dependencies of a project using ghc 8.8.4.
-So you can add that to the relevant GC root.
-In practice, if you're using a CI system like Hydra/Hercules, this means adding it to a job in `release.nix`/`ci.nix`.

docs/tutorials/getting-started.md

@@ -8,29 +8,21 @@ project and its dependencies into Nix code.
 Assuming you have [Nix](https://nixos.org/download.html) installed, you can
 start setting up your project.
 
-## Setting up the Cachix binary cache
+## Setting up the binary cache
 
-You can **avoid compiling GHC and nix-tools** by configuring
-[Cachix](https://cachix.org) so you can benefit from the binary cache built by
-CI:
+IMPORTANT: you *must* do this or you *will* build several copies of GHC!
 
-```bash
-$ nix-env -iA cachix -f https://cachix.org/api/v1/install
-installing 'cachix-0.3.8'
-building '/nix/store/bh176xhpk4wrjm56iahm86wf85jaz23v-user-environment.drv'...
-created 42 symlinks in user environment
+You can configure Nix to use our binary cache, which is pushed to by CI, so should contain the artifacts that you need.
 
-$ cachix use iohk
-Configured https://iohk.cachix.org binary cache in ~/.config/nix/nix.conf
-```
-
-Note: `haskell.nix` currently uses multiple CI providers to build derivations and store outputs. To improve your chances of getting a cache hit, you might want to add the following additional substituter to `~/.config/nix/nix.conf`:
+You need to add the following sections to `/etc/nix/nix.conf` or, if you are a trusted user, `~/.config/nix/nix.conf` (if you don't know what a "trusted user" is, you probably want to do the former).
 
 ```
 trusted-public-keys = [...] hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= [...]
 substituters = [...] https://hydra.iohk.io [...]
 ```
 
+This can be tricky to get setup properly. If you're still having trouble getting cache hits, consult the corresponding [troubleshooting section](../reference/troubleshooting#why-am-i-building-ghc).
+
 ## Scaffolding
 
 The following work with `stack.yaml` and `cabal.project` based