add an "about this guide" section

Author: nikomatsakis

src/SUMMARY.md

@@ -1,5 +1,6 @@
 # Summary
 
+- [About this guide](./chap-000-about-this-guide.md)
 - [How to build the compiler and run what you built](./chap-010-how-to-build-and-run.md)
 - [Using the compiler testing framework](./chap-020-running-tests.md)
 - [Walkthrough: a typical contribution](./chap-030-walkthrough.md)

src/chap-000-about-this-guide.md

@@ -0,0 +1,14 @@
+# About this guide
+
+This guide is meant to help document how rustc -- the Rust compiler --
+works, as well as to help new contributors get involved in rustc
+development. It is not meant to replace code documentation -- each
+chapter gives only high-level details, the kinds of things that
+(ideally) don't change frequently.
+
+The guide itself is of course open source as well, and the sources can
+be found at [the GitHub repository]. If you find any mistakes in the
+guide, please file an issue about it -- or, even better, open a PR
+with a correction!
+
+[the GitHub repository]: https://github.com/rust-lang-nursery/rustc-guide/