add a proper README

Author: SethTisue

README.md

@@ -1,11 +1,65 @@
-## Status
+# scala-library-next
 
-This repo is brand new (September 2020). We're still putting everything in place.
+This repo holds code that will be added to a future version of
+the Scala standard library.
 
-This hasn't been published yet. It isn't even ready for contributions yet. Stay tuned.
+We will publish this as a library, to make the additions usable
+immediately. But publishing hasn't happened yet.  (The repo only just
+started, in October 2020.)
 
-## Purpose and scope
+## Why?
+
+Why make a library for this?  Because:
+
+* Scala 2.13 is expected to be final major release of Scala 2.
+* We cannot make additions to the standard library in the Scala 2.13.x
+  series, because of our [binary compatibility
+  constraints](https://docs.scala-lang.org/overviews/core/binary-compatibility-of-scala-releases.html).
+* To help users transition from Scala 2 to 3, Scala 2.13 and 3.0 will
+  share the same standard library.
+
+Therefore, additions to the library cannot happen until Scala 3.1 at the earliest.
+
+But why publish the additions separately in the meantime?  Because:
+
+* Users may find the additions useful now.
+* Having users start using the additions now will help shake out
+  problems early.
+* Contributors will be more motivated to work on improvements if users
+  can use them now.
+
+## What can be added?
+
+Anything merged here _will_ become part of the next Scala standard library.
+
+Therefore, we will not merge anything here unless the Scala 2 and 3
+teams agree on the addition.  The bar for accepting additions remains
+very high.
+
+Is it okay to open an issue and/or pull request regardless? Yes,
+definitely. Let's discuss your idea. Just be aware that the bar is
+high and contributions may be rejected unless there is a high degree
+of consensus and confidence that it really belongs in the standard
+library.
+
+It's not required, but you may wish to bring your idea up on
+[contributors.scala-lang.org](https://contributors.scala-lang.org)
+first to gauge reaction.
+
+There are may be technical constraints on what can be added, since
+this a separate codebase from the actual standard library.  So for
+example if you want to add a new method to an existing class, it must
+be added as an extension method. We are still discussing details on
+[issue #4](https://github.com/scala/scala-library-next/issues/4).
+
+## History
+
+The discussions that led to this repo being created are here:
 
 * https://github.com/scala/scala-dev/issues/661
 * https://github.com/scala/scala-collection-compat/issues/329
 * https://github.com/scala/scala-dev/issues/449
+
+The name was discussed here:
+
+* https://github.com/scala/scala-library-next/issues/1