Clarify how to run tests for contributors

CONTRIBUTING.md

@@ -35,13 +35,11 @@ which in turns depends on some common libraries, in particular worth mentioning:
 
 Source code is located at `lib/assets/javascripts`, dependencies at `vendor/assets/javascripts`.
 
-Run `grunt jasmine` to generate `/cartodb/_SpecRunner.html`.
-
 See [doc/frontend.md](doc/frontend.md) for more in-depth documentation.
 
 Until our guidelines are publically available follow the existing file/directory and style structure.
 
-### Writing testcases
+### Writing & running tests
 
 Tests reside in the `lib/assets/test` directory. We use
  - [Jasmine 2.1](jasmine.github.io/2.1/introduction.html) as test framework
@@ -53,6 +51,20 @@ if you're writing tests for current code or the newer browserify modules).
 
 Until our guidelines are publically available follow the existing file/directory and style structure.
 
+All tests can be run by
+```bash
+grunt jasmine
+```
+
+This task generates a (non-versioned) file ./_SpecRunner.html which you can open in the browser to easier debug your code and tests.
+If you only want to run a subset of tests open this file in the browser and add the querystring
+`?spec=start-of-describe`, e.g.
+```
+# In your browser open (don't forget to change /path/to/cartodb ;):
+file:///path/to/cartodb/_SpecRunner.html?spec=cdb.admin.User
+```
+
+
 ## CSS
 
 We use [SASS](http://sass-lang.com/),