Highlight that we require nightly in the README

Author: nrc

README.md

@@ -5,6 +5,8 @@ A tool for formatting Rust code according to style guidelines.
 
 ## How to use
 
+You'll need a pretty up to date version of the **nightly** version of Rust.
+
 `cargo build` to build.
 
 `cargo test` to run all tests.
@@ -15,7 +17,6 @@ to run on the top file. You'll probably want to set the `WriteMode` in the call
 to `run` in `main()`. Eventually you should be able to set the mode from the
 command line or from a config file or something.
 
-You'll need a pretty up to date version of the nightly version of Rust.
 
 ## Use cases