Add instructions to edit only files under src folder

Author: zenorocha

CONTRIBUTING.md

@@ -2,8 +2,10 @@
 
 1. Fork it!
 2. Create your feature branch: `git checkout -b my-new-feature`
-3. Commit your changes: `git commit -am 'Add some feature'`
+3. Commit your changes: `git commit -m 'Add some feature'`
 4. Push to the branch: `git push origin my-new-feature`
 5. Submit a pull request :D
 
+Don't edit files under `dist` folder, those are generated after running `grunt` command, edit files under `src` folder instead.
+
 Also remember to follow [jQuery's Code Style](http://contribute.jquery.org/style-guide/js/).