Reminder about REPO variable

CONTRIBUTING.md

@@ -3,7 +3,10 @@ We love pull requests. Here's a quick guide:
 1. Fork the repo.
 
 2. Run the tests. We only take pull requests with passing tests, and it's great
-to know that you have a clean slate: `bundle && rake`
+to know that you have a clean slate: `REPO=path/to/.git bundle && rake`.
+
+Note that you must include the `REPO` environment variable pointing at your local
+`.git` repository in order to generate the test application required by tests.
 
 3. Add a test for your change. Only refactoring and documentation changes
 require no new tests. If you are adding functionality or fixing a bug, we need
@@ -35,4 +38,4 @@ Syntax:
 * a = b and not a=b.
 * Follow the conventions you see used in the source already.
 
-And in case we didn't emphasize it enough: we love tests!
+And in case we didn't emphasize it enough: we love tests!