doc: add contribution guide

CONTRIBUTING.md

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+# Contributing
+
+This repository is the official **community maintained** helm chart for oauth2-proxy and is not to be confused with the helm chart published by bitnami. We rely on you to test your changes sufficiently.
+
+## Pull Requests
+
+All submissions, including submissions by project members, require review. We use GitHub pull requests for this purpose.
+
+### Pull Request Title
+
+We do not enforce the title of your pull request to follow guidelines but we do appreciate [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/).
+
+## Documentation
+
+The documentation in the chart [README.md](charts/oauth2-proxy) and the `values.yaml` should always be kept up to date if you make changes to existing parameters or introduce new ones.
+
+### Artifact Hub Annotation
+
+Since we release the chart on Artifact Hub we require you to update the `artifacthub.io/changes` annotation in the `Chart.yaml`.
+
+* [https://artifacthub.io/docs/topics/annotations/helm/](https://artifacthub.io/docs/topics/annotations/helm/)
+
+
+## Versioning
+
+We follow the [semver standard](https://semver.org/) for the chart version and application version.
+
+Always consider your changes and try to avoid breaking changes where possible.
+
+### New Application Versions
+
+The application version is only to be updated if a new release of the oauth2-proxy application repo was published.
+
+### Immutability
+
+Each release must be immutable. Any change to a chart (even just documentation) requires a version bump. Trying to release the same version twice will result in an error.
+
+
+## Testing
+
+When making changes to the logic or resources of the chart please make sure you tested those changes in two ways:
+
+* Existing helm release with the chart version before your changes: `helm upgrade`
+* Fresh helm release with you changes: `helm install`
+
+
+### Testing Charts
+
+As part of the Continuous Integration system we run Helm's [Chart Testing](https://github.com/helm/chart-testing) tool.
+
+The checks for Chart Testing are stricter than the standard Helm requirements.
+
+The configuration can be found in [ct.yaml](ct.yaml)
+
+If you have `ct` installed you can manually invoke the linting with the following command:
+
+```shell
+ct lint --config ct.yaml
+```
+
+If you want to run the tests locally we recommend to use [kind](https://kind.sigs.k8s.io).
+
+Prerequisites:
+
+```shell
+# Add monitoring CRD
+kubectl apply --server-side -f https://raw.githubusercontent.com/prometheus-operator/prometheus-operator/release-0.78/example/prometheus-operator-crd/monitoring.coreos.com_servicemonitors.yaml
+```
+
+Run the tests:
+
+```shell
+ct --config ct.yaml install
+```
+
+## Publishing Changes
+
+Changes are automatically publish whenever a commit is merged to the `main` branch by the CI job (see `./.github/workflows/release.yml`).