Suggest + volunteer: add CONTRIBUTING.md

[richelbilderbeek]

From, for example, 'Best Practices for Maintainers' on can learn it is a good idea to have guidelines on the rules for contributors to do so, e.g. one of my own CONTRIBUTING.md documents. One of the many benefits of this, is that is makes it easier to say 'no' to undesired features.

I suggest to add such a CONTRIBUTING.md document and volunteer to create a first sketch of one. Of course, the current maintainers are boss, so I do not expect the rules I put in to become the actual rules :-)

Good idea?