document existing setup

Author: jyn514

src/doc/unstable-book/README.md

@@ -0,0 +1,8 @@
+# Unstable Book
+
+These are the sources for <https://doc.rust-lang.org/nightly/unstable-book/>.
+To generate them, run `./x doc unstable-book`, which will generate HTML files in `build/host/doc/unstable-book` using `src/tools/rustbook`.
+If you need to change the overall structure, modify `src/tools/unstable-book-gen/src/SUMMARY.md`.
+
+Note that most of this book is autogenerated by `unstable-book-gen`, with the exception of `compiler-flags` and `compiler-environment-variables`.
+As a result, it does not integrate well with `mdbook`. Use `./x doc` instead.