Create CONTRIBUTING.md

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:
+
+* 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.
+* Run tests with:
+
+        python2 setup.py test
+        python3 setup.py test
+    
+  Obviously, I won't accept anything that makes the tests fail. When you submit a Pull Request, Travis CI will
+  automatically run all tests for various Python versions, but it's better if you run the tests locally first.
+  
+  Make sure you have the `mock` package installed if you test on a Python version older than 3.3.
+* Add tests for your change(s):
+  * Bugfixes: add a testcase that covers your bugfix, so the bug won't happen 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.
+
+### 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.