Add contributing guidelines

[agbeltran]

Add a file CONTRIBUTING.md explaining how to contribute to MOD.

[agbeltran]

for inspiration, let's consider
https://github.com/geneontology/go-ontology/blob/master/CONTRIBUTING.md

https://github.com/codemeta/codemeta/blob/master/CONTRIBUTING.md

https://github.com/dgarijo/Widoco/blob/master/CONTRIBUTING.md

[agbeltran]

We have a first version of the CONTRIBUTING guidelines pointing to the discussions on design issues. Leaving the issue open as we will add more details as we progress with the work.

[jonquet]

Assign to me too to review guidelines