Contributing: add grunt command for tests [ci-skip]

contributing.md

@@ -175,16 +175,19 @@ license your work under the [MIT License](http://en.wikipedia.org/wiki/MIT_Licen
 4. Don't manually update the version number in `package.json`. This is done using a Grunt task on deployment.
 
 <a name="grunt"></a>
-#### Using CoffeeScript and Grunt
+#### Grunt tasks: Running Tests and using CoffeeScript
 
 
 To install all development dependencies, in the project's root directory, run
 
     npm install
 
-Once you're configured, building the JavaScript from the command line is easy:
+Once you're configured, `grunt` tasks are available:
+
+    grunt test                 # run the tests in spec/ from CoffeeScript source
+
+    grunt build                # build Chosen from CoffeeScript source
 
-    grunt build                # build Chosen from source
     grunt watch                # watch coffee/ for changes and build Chosen
 
 If you're interested, you can find the task in [Gruntfile.coffee](https://github.com/harvesthq/chosen/blob/master/Gruntfile.coffee).