Add a note about running jshint in the CONTRIBUTING file

CONTRIBUTING.md

@@ -6,3 +6,12 @@ This contributors document should be the canonical place for answers to:
     What is the code review process?
     What sort of organizational schema is used for Github Issues? What labels are used, and what do they mean? How are milestones handled?
 
+# Before submitting a pull request
+
+To install jshint, simply `npm install jshint` from the root directory of the project. Then run like it like this:
+
+    node_modules/.bin/jshint data/*.js
+
+and make sure your changes are not adding any new warnings or errors.
+
+Also keep an eye on the command line warnings and errors when running the extension via `cfx run`.