improve the README

Author: nikomatsakis

README.md

@@ -1,3 +1,25 @@
-An attempt to document how the compiler works. The intention is to
-bootstrap this "in repo" and eventually move the text over into the
-main rustc repo.
+This is a collaborate effort to build a guide that explains how rustc
+works. The aim of the guide is to help new contributors get oriented
+to rustc, as well as to help more experienced folks in figuring out
+some new part of the compiler that they haven't worked on before.
+
+The guide can be useful today, but it has a lot of work still go.
+Once it gets more complete, the plan is probably to move it into the
+[main Rust repository](https://github.com/rust-lang/rust/).
+
+### Contributing to the guide
+
+If you'd like to help finish the guide, we'd love to have you! The
+main tracking issue for the guide
+[can be found here](https://github.com/rust-lang-nursery/rustc-guide/issues/6). From
+there, you can find a list of all the planned chapters and subsections
+-- if you think something is missing, please open an issue about it!
+Otherwise, find a chapter that sounds interesting to you and then go
+to its associated issue. There should be a list of things to do.
+
+**In general, if you don't know how the compiler works, that is not a
+problem!** In that case, what we will do is to schedule a bit of time
+for you to talk with someone who **does** know the code, or who wants
+to pair with you and figure it out.  Then you can work on writing up
+what you learned.
+