Establish rolling sponsorship contracts

[meijer]

Guido to work on "REFEDS-style" construction to have an easier and more consistent way to allow organisations for an easier and more consistent way to annually contribute funds

[meijer]

First 6 annual contribution contracts are now active.

[meijer]

Work left: put template in place, and put overview out there on the github/website which organisations have committed to an annual contribution.

[meijer]

This is actually progressing nicely, with currently 11 active contributor agreements. Work left: put template in repository, and create separate template for "sponsor" and "contribution".