Documentation: add CONTRIBUTING.MD (backport #310)

CONTRIBUTING.md

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+# Contributing to cephadm-ansible
+
+1. Follow the [commit guidelines](#commit-guidelines)
+
+## Commit guidelines
+
+- All commits should have a subject and a body
+- The commit subject should briefly describe what the commit changes
+- The commit body should describe the problem addressed and the chosen solution
+  - What was the problem and solution? Why that solution? Were there alternative ideas?
+- Wrap commit subjects and bodies to 80 characters
+- Sign-off your commits
+- Add a best-effort scope designation to commit subjects. This could be a directory name, file name,
+  or the name of a logical grouping of code. Examples:
+  - library: add a placeholder module for the validate action plugin
+  - site.yml: combine validate play with fact gathering play
+- Commits linked with an issue should trace them with :
+  - Fixes: #2653
+
+[Suggested reading.](https://chris.beams.io/posts/git-commit/)
+
+## Pull requests
+
+### Jenkins CI
+
+We use Jenkins to run several tests on each pull request.
+
+If you don't want to run a build for a particular pull request, because all you are changing is the
+README for example, add the text `[skip ci]` to the PR title.
+
+### Merging strategy
+
+Merging PR is controlled by [mergify](https://mergify.io/) by the following rules:
+
+- at least one approuval from a maintainer
+- a SUCCESS from the CI pipeline "cephadm-ansible PR Pipeline"
+
+If you work is not ready for review/merge, please request the DNM label via a comment or the title of your PR.
+This will prevent the engine merging your pull request.
+
+### Backports (maintainers only)
+
+If you wish to see your work from 'main' being backported to a stable branch you can ping a maintainer
+so he will set the backport label on your PR. Once the PR from main is merged, a backport PR will be created by mergify,
+if there is a cherry-pick conflict you must resolv it by pulling the branch.
+
+**NEVER** push directly into a stable branch, **unless** the code from main has diverged so much that the files don't exist in the stable branch.
+If that happens, inform the maintainers of the reasons why you pushed directly into a stable branch, if the reason is invalid, maintainers will immediatly close your pull request.
+
+### Keep your branch up-to-date
+
+Sometimes, a pull request can be subject to long discussion, reviews and comments, meantime, `main`
+moves forward so let's try to keep your branch rebased on main regularly to avoid huge conflict merge.
+A rebased branch is more likely to be merged easily & shorter.
+
+### Organize your commits
+
+Do not split your commits unecessary, we are used to see pull request with useless additional commits like
+"I'm addressing reviewer's comments". So, please, squash and/or amend them as much as possible.
+
+Similarly, split them when needed, if you are modifying several parts in cephadm-ansible or pushing a large
+patch you may have to split yours commit properly so it's better to understand your work.
+Some recommandations:
+
+- one fix = one commit,
+- do not mix multiple topics in a single commit,
+- if you PR contains a large number of commits that are each other totally unrelated, it should probably even be split in several PRs.
+
+If you've broken your work up into a set of sequential changes and each commit pass the tests on their own then that's fine.
+If you've got commits fixing typos or other problems introduced by previous commits in the same PR, then those should be squashed before merging.
+
+If you are new to Git, these links might help:
+
+- [https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History)
+- [http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html](http://gitready.com/advanced/2009/02/10/squashing-commits-with-rebase.html)