Move contributing instructions to CONTRIBUTING.md

[pquentin]

As suggested by @dakrone in #3926 (comment). The information was mostly in the pull request template, but that's arguably a bit too late.

[dakrone]

I know that the default target for make displays the help, but perhaps it could be changed to do all the necessary bits prior to a PR being submitted? A lot of folks are used to default make building as necessary (I know we have no make install equivalent for this).

No pressure, just a suggestion from my side!

[pquentin]

Thank you for the suggestion. Changing make to make setup && make install could be interesting, but we probably need to make it fast to detect an existing install. For now, I have decided to merge what I had since it's a step in the right direction.