Add how to run tidy script in isolation to text

This is already mentioned elsewhere, but important things are repeated twice.

Closes #124

Author: gnzlbg

src/conventions.md

@@ -11,7 +11,8 @@ rustc is slowly moving towards the [Rust standard coding style][fmt];
 at the moment, however, it follows a rather more *chaotic* style.  We
 do have some mandatory formatting conventions, which are automatically
 enforced by a script we affectionately call the "tidy" script.  The
-tidy script runs automatically when you do `./x.py test`.
+tidy script runs automatically when you do `./x.py test` and can be run 
+in isolation with `./x.py test src/tools/tidy`.
 
 [fmt]: https://github.com/rust-lang-nursery/fmt-rfcs