adding description of guard to README

Author: chadoh

README.md

@@ -320,8 +320,16 @@ And then, install all your dependencies:
 
 You need to have an instance of MongoDB running on your computer or all the mongo tests will fail miserably.
 
+To run all the tests once, simply use
+
     rake test
 
+while you're developing, it is recommended that you run
+
+    bundle exec guard
+
+to have the relevent test run every time you save a file.
+
 ## Integration tests
 
 If you make changes or add features to the jQuery part, please make sure