add guidelines for contributing to the project

[pbellec]

this project has exemplary guidelines
https://tedana.readthedocs.io/en/latest/

[ltetrel]

This is definitively important in doc.
for now I just added a small text here: https://docs.neurolibre.org/en/latest/FAQ.html#i-want-to-contribute-how-can-i-do-that
I think as of now there are more important matter for the doc, when we will be in beta we can move this issue.