pre-push hook to run tests

Adds a note on how to use Git hooks to the README.

README.md

@@ -5,4 +5,13 @@ Privacy Badger for Firefox
 
 1.  Install addon-sdk [github](https://github.com/mozilla/addon-sdk)
 2.  [Addon-sdk documentation] (https://developer.mozilla.org/en-US/Add-ons/SDK/Tutorials/Installation)
-2.  cfx run
+3.  cfx run
+
+Contributing
+============
+
+Git hooks
+---------
+
+Are in `hooks/`. You can use them by copying the files to `.git/hooks`. The
+pre-push hook will run the unit tests, and cancel the push if they fail.

hooks/pre-push

@@ -0,0 +1,12 @@
+#!/bin/bash
+
+# Pre-push hook. If you want to test with a different version of firefox, put
+# the path in the CFX_FIREFOX environment variable.
+
+cmd='cfx test'
+if [ -n "$CFX_FIREFOX" ];
+then
+  cmd="$cmd -b $CFX_FIREFOX"
+fi
+echo $cmd
+$cmd