feat: Add CONTRIBUTING.md

Signed-off-by: Anthony TREUILLIER <anthony.treuillier@scality.com>

Author: anthony-treuillier-scality

CONTRIBUTING.md

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+# Contributing to the Project
+
+This document contains and defines the rules that have to be followed by any
+contributor to the project, in order for any change to be merged into the
+stable branches.
+
+## Workflow Guidelines
+
+### Branching Guidelines
+
+When branching, the branch name should have one of the following prefixes:
+
+* improvement/
+* bugfix/
+* feature/
+
+Additionally, the branch name should also contain a reference to the issue
+being tackled. For example, a branch tackling issue 123 introducing the new
+feature FooBar might be named `feature/GH-123-add-shiny-new-feature-FooBar`.
+
+
+### Committing Guidelines
+
+No restrictions are placed at this time on individual commits passing in the
+CI and/or maintaining full functionality of the repository.
+
+Commit messages should:
+
+* have a short (< 50 characters) summary as title
+* contain more explanations, if necessary, in the body
+* contain a reference to the issue being tackled in the body
+
+A commit message should *not* contain a reference to the issue in the title.
+
+### Pull Request Guidelines
+
+Pull requests should contain in their body a reference to the GitHub issue 
+being targeted by the changeset introduced.
+
+### Signing your work
+
+In order to contribute to the project, you must sign your work. By signing your
+work, you certify to the statements set out in the Developer Certificate of
+Origin ([developercertificate.org](https://developercertificate.org/))
+
+Signing your work is easy. Just add the following line at the end of each of
+your commit messages. You must use your real name in your sign-off.
+
+```
+Signed-off-by: Jane Doe <jane.doe@email.com>
+```
+
+If your `user.name` and `user.email` are set in your git configs, you can sign
+each commit automatically by using the `git commit -s` command.
+
+## Reporting an issue
+
+This project uses Github issues to manage the issues.
+
+Before creating an issue:
+
+1. upgrade the operator to the latest supported release version, and check whether your bug is still present,
+2. ensure the operator version is supported by the PowerDNS version you are using,
+3. have a look in the opened issues if your problem is already known/tracked, and possibly contribute to the thread with your own information.
+
+If none of the above was met, open an issue directly in Github, select the appropriate issue template and fill-in each section when applicable.
+
+## Testing & Linting
+
+Run tests and lint the code:
+```go
+go test -v ./...
+golangci-lint run
+```