Add or link to contributing guidelines

[Kevin-Mattheus-Moerman]

Add contributing guidelines e.g. in the form of a CONTRIBUTING.md file, and link to it in the readme.

Here is an example for GIBBON: https://github.com/gibbonCode/GIBBON/blob/master/CONTRIBUTING.md

Here is another template: https://gist.github.com/PurpleBooth/b24679402957c63ec426

[SteveMaas1978]

CONTRIBUTING.md file was added to repo.