Add CONTRIBUTING.md

Signed-off-by: Marco Franssen <[email protected]>

CONTRIBUTING.md

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+# CONTRIBUTING
+
+:tada: Thanks for your interest in contributing to this project. In this document we outline a few guidelines to ease the way your contributions flow into this project.
+
+## Contributor guidelines and Governance
+
+Please see [CONTRIBUTING](https://github.com/spiffe/spiffe/blob/main/CONTRIBUTING.md) and [GOVERNANCE](https://github.com/spiffe/spiffe/blob/main/GOVERNANCE.md) from the SPIFFE project.
+
+In addition to those guidelines we also have a couple guidelines specifically tailored to make your contributions as smooth as possible to contribute Helm charts.
+
+## Prerequisites
+
+- **Helm 3** or higher (<https://helm.sh/docs/intro/install/>)
+
+## Commit style
+
+Ensure you have clear and concise commits, written in the present tense. See [Kubernetes commit message guidelines](https://www.kubernetes.dev/docs/guide/pull-requests/#commit-message-guidelines) for a more detailed explanation of our approach.
+
+```diff
++ git commit -m "Bump spire appVersion 1.4.4"
+- git commit -m "Bumped spire appVersion 1.4.4"
+```
+
+## PRs
+
+Stick with one feature/chart per branch. This allows us to make small controlled releases of the charts without blocking other chart releases in case of issues releasing a chart.
+
+Ensure your branch is rebased on top of main before issuing your PR. This to keep a clean Git history and to ensure your changes are working with the latest main branch changes.
+
+```bash
+git checkout main
+git pull
+git checkout «your-branch»
+git rebase main
+```
+
+## Generating documentation
+
+Any changes to Chart.yaml or values.yaml require an update of the README.md. This update can easily be generated using [helm-docs][].
+
+```shell
+helm-docs -g charts/«chart-name» -t .github/README.md.tmpl
+```
+
+[helm-docs]: https://github.com/norwoodj/helm-docs "Generate documentation for your Helm chart."