Add section on 'coverage', restructured document a bit.

CONTRIBUTING.md

 If you're reading this, you may have interest in enhancing topydo. Thank you!
 
 Please read the following guidelines to get your enhancement / bug fixes
-smoothly into topydo:
+smoothly into topydo.
+
+### General
 
 * This Github page defaults to the **stable** branch which is for **bug fixes
   only**. If you would like to add a new feature, make sure to make a Pull
   Request on the `master` branch.
+
+* Use descriptive commit messages.
+
+### Coding style
+
+* Please try to adhere to the coding style dictated by `pylint` as much
+  possible. I won't be very picky about long lines, but please try to avoid
+  them.
+* I strongly prefer simple and short functions, doing only one thing. I'll
+  request you to refactor functions with massive indentation or don't fit
+  otherwise on a screen.
+
+### Testing
+
 * Run tests with:
 
         ./run-tests.sh [python2|python3]
@@ -21,13 +37,17 @@ smoothly into topydo:
     ever again.
   * Features: add testcases that checks various inputs and outputs of your
     feature. Be creative in trying to break the feature you've just implemented.
-* Use descriptive commit messages.
+* Check the test coverage of your contributed code, in particular if you
+  touched code in the topydo.lib or topydo.command packages:
 
-### Coding style
+      pip install coverage
+      coverage run setup.py test
+      coverage report
+
+  Or alternatively, for a more friendly output, run:
+
+      coverage html
+
+  Which will generate annotated files in the *htmlcov* folder. The new code
+  should be marked green (i.e. covered).
 
-* Please try to adhere to the coding style dictated by `pylint` as much
-  possible. I won't be very picky about long lines, but please try to avoid
-  them.
-* I strongly prefer simple and short functions, doing only one thing. I'll
-  request you to refactor functions with massive indentation or don't fit
-  otherwise on a screen.