docs: adds Contribution Flow section to better manage expectations

Would you like to contribute? There are 2 ways:

1. Fork this project, commit your changes there, and do a Merge Request against

   this project. Don't forget to assign either @falconierif or @hoverht to have

   a look at your code.

   this project. Don't forget to assign either @falconierif or @hoverht as

   reviewer to have a look at your code.

2. Ask us to become a Developer in this repo, so you can develop in a branch

   here.

## Contribution Flow

1. Contributor opens a merge request with a

   [conventional commit title](https://www.conventionalcommits.org/en/v1.0.0/#summary)

   and a clear summary of the change proposed. By default the title should start

   with `Draft: `.

2. When the merge request is ready (Pipelines are all finished and green), the

   contributor should `Mark as ready` (= title no longer starts with `Draft: `),

   and assign a reviewer.

3. Reviewer should directly implement simple feedback in a separate merge

   request to be merged into the merge request under review.

   - When feedback is much more complex, or discussion is needed, reviewer

     should not implement suggested changes to avoid unnecessary work and

     unnecessary cognitive load on all parties. First discuss the feedback in

     the comments, then discuss the what/how/who of implementing changes.

4. The philosophy should be to move fast and break things, that's why we pin

   versions.

### pre-commit hooks

We use pre-commit hooks in this project. You don't have to use them, but if you