Add documentation to "contribution" part of README

[widhalmt]

We have some rules in README.md. We should add, that every pull request must come with documentation for the change it introduces. Now is the time where we see more undocumented variables etc. .

And, as @xtruthx just experienced (sorry again for that), some more sophisticated parts of the code aren's as well documented as we all wished. So while using the collection seems to be quite straight forward, changes are hard because the more complex parts aren't easy to comprehend. Sometimes even usage is hard: See #218 for example.