[Policy] Beta cycle contribution Policy

As beta is around the corner it's a good opportunity to have an update to the contributing policy to ease the work and have the project reach stable faster!

Here is a contributing.md update:

### Issue

- When fork an issue, use a short branch meaningfull name with proper wording (no article, no concatenated words, ie: no 300000-mytitleislongandthisbran )
- When a PR is created it MUST be set DRAFT immediately until the issue is in `Need review`

### Pull requests

We accept only pull requests (PR), no patches, with the following expectations:

- Maintain the existing code style, CI pass is mandatory. ⚠️ Please ensure your PR is all green on CI before asking for review
- Are focused on a single change (i.e. avoid large refactoring or style adjustments in untouched code if not the primary goal of the pull request) from a single Drupal issue
- Contributor is responsible for rebase and MUST do it before review, exception on Draft that require review

#### Code

- Unless typo or hotfix, any change MUST be covered by a test
- Module MUST maintain install / uninstall and update workflow
- Trait MUST not included anything external, if so it's not a Trait, exception for clas that can be instantiated (translation, url,...)

#### Review

- All comments on code MUST be done with the Gitlab comment system to allow `resolve`
- All remarks MUST be `resolved` before merging
- If an issue or MR is not resolved, maintainers can allow a reasonable time to reach consensus, if not resolved the lead have final decision

#### Commit

- Use new Drupal contribution record system