doc: document documentation build, mention fork

doc/SUMMARY.md

@@ -18,4 +18,5 @@
   - [Ubuntu Setup](./make/ubuntu-16.04.md)
   - [macOS Setup](./make/osx-10.9.md)
   - [Windows Setup](./make/msys2.md)
+- [Building This Manual](./mdbook.md)
 - [Fixing Tests](./fixing_tests.md)

doc/mdbook.md

@@ -0,0 +1,14 @@
+This manual is generated by [mdBook](https://github.com/rust-lang/mdBook). We are currently using a
+[fork](https://github.com/leanprover/mdBook) of it for the following additional features:
+
+* Add support for hiding lines in other languages [#1339](https://github.com/rust-lang/mdBook/pull/1339)
+
+To build this manual, first install the fork via
+```
+cargo install --git https://github.com/leanprover/mdBook mdbook
+```
+Then use e.g. [`mdbook watch`](https://rust-lang.github.io/mdBook/cli/watch.html) in the `doc/` folder:
+```
+cd doc
+mdbook watch --open  # opens the output in `out/` in your default browser
+```