Update PR instructions in CONTRIBUTING.md

[jcailler]

Add git instruction for contributors

• How to create a branch
• How to create a pull request
• Random useful tips

[jrosain]

FYI as the go source code has not been modified, we don't need to run the full CI on this PR.

(see e.g., on the maintainer section of the contributing file)

[jrosain]

To be honest, I'm not sure it's our job to give these tips. I'm not against contributors having their cheatsheet, or even linking to a github command cheatsheet inside the contribution file, e.g., this one for instance.

My initial thoughts: I don't think we should keep the "create a branch" section, but maybe we can keep the creation of a PR / rebasing comments by factoring them through a specific section for pull requests. But the text would need to be reworked before. For instance, you should push your branch, then open a PR.
Squashing the commits is not mandatory as we might want to keep the history in some cases (see e.g., #26). We should precise that the maintainer will tell whether to squash the commits or not, so that you don't have to worry about that when opening a PR. We should definitely keep the rebase command and tell that before opening a PR, a branch should either be up-to-date from upstream/master or mergeable without conflicts (even though this is already said in the general informations, we could provide the commands).

[jrosain]

I'm converting this PR to draft until further discussions / progress.

[jrosain]

Your branch seems to be in a strange state with the merge commit and everything.

[jrosain]

I feel like a transition is missing between these two paragraphs. Maybe something like: "If you want to open a pull request, you should roughly follow these steps:"