New governance rules

Considering the importance of the merge requests by contributors and that I only have 24 hours a day, I think it's time to give the push access on master to more people. It's been some time I have started thinking about it but there's a hard leap of faith to carry out here. I am willing to give away a bit of power for a faster MR turnover (and less stress for myself).

I am thinking about giving the push access to @ZeHiro @nicofrand and @phyks at the moment, considering their impressive and numerous contributions to this project. Thank you for that! (Of course, you don't have to accept if you don't want to)

This MR changes the CONTRIBUTING.md file to be more explicit about some things that were implicit so far (BDFL, merge/review rules), and some rules that look a bit authoritative, but are a first step to scattering power. I am willing to change this over time, and I would like for us to get to an agreement. Also, let's not take it too seriously and keep it to the minimum (unless you think we need more, in which case I'm happy to hear why).

Comments appreciated on this MR. If/when accepted and agreed, I'll give the Master role to the three persons mentioned above 🥂