Improve the CONTRIBUTING.md

[curquiza]

The goal of this issue is to make the CONTRIBUTING.md less "heavy" for the contributors. Indeed, the more the file is big, the less they are going to read it.

Here is what I suggest:

• the Release Process section was in the Development Workflow: I suggest to move it to a separated section and to specify this section concerns only the Administrators, I mean the Meili team. If the contributors are curious, they can read. If they don't care, they know they can skip this section.
• Add a Bors related section, but the section is really small and redirects to the Bors guide I created: the goal is to mention it but not to make the CONTRIBUTING file too heavy. As this section only concerns the Meili team, adding a redirection is not impactful.
• Make the Automated Changelogs section smaller: as for the Bors section, the purpose is to reduce the CONTRIBUTING.md file and to redirect to the release-drafter guide. See Add release-drafter guide #49.

I created a PR so that you can approve on it! #50

I know a lot other things could be improved, this is just the first steps of a general change 😉

[curquiza]

Done!