Add a note about Rust 0.9 support.

Author: chris-morgan

README.rst

@@ -13,6 +13,15 @@ This project has two parts:
 Both are in progress; both have basic, low-level implementations in place.
 Neither is complete nor yet compliant in any way.
 
+Rust versions
+-------------
+
+I urge you to track Rust master as rust-http does, but if you really are set on
+using Rust 0.9, you can use the [`rust-0.9-compatible`
+branch](https://github.com/chris-morgan/rust-http/commits/rust-0.9-compatible).
+It is not maintained, however; it's just the last commit that *will* work on
+Rust 0.9.
+
 Goals
 -----