Improve contribution part of this repository (#328)

Reviewed-on: https://gitea.com/gitea/helm-chart/pulls/328
Reviewed-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: justusbunsi <justusbunsi@noreply.gitea.io>
Co-committed-by: justusbunsi <justusbunsi@noreply.gitea.io>

CONTRIBUTING.md

@@ -0,0 +1,52 @@
+# Contribution Guidelines
+
+Any type of contribution is welcome; from new features, bug fixes, tests,
+refactorings for easier maintainability or documentation improvements.
+
+## Development environment
+
+- [`node`](https://nodejs.org/en/) at least current LTS
+- [`helm`](https://helm.sh/docs/intro/install/)
+- `make` is optional; you may call the commands directly
+
+When using Visual Studio Code as IDE, following plugins might be useful:
+
+- [Markdown All in One](https://marketplace.visualstudio.com/items?itemName=yzhang.markdown-all-in-one)
+- [markdownlint](https://marketplace.visualstudio.com/items?itemName=DavidAnson.vscode-markdownlint)
+- [Helm Intellisense](https://marketplace.visualstudio.com/items?itemName=Tim-Koehler.helm-intellisense)
+
+## Documentation Requirements
+
+The `README.md` must include all configuration options. The parameters section
+is generated by extracting the parameter annotations from the `values.yaml` file,
+by using [this tool](https://github.com/bitnami-labs/readme-generator-for-helm).
+
+If changes were made on configuration options, run `make readme` to update the
+README file.
+
+## Pull Request Requirements
+
+When submitting or updating a PR:
+
+- make sure it passes CI builds.
+- do not make independent changes in one PR.
+- try to avoid rebases. They make code reviews for large PRs and comments much harder.
+- if applicable, use the PR template for a well-defined PR description.
+- clearly mark breaking changes.
+
+## Local development & testing
+
+For local development and testing of pull requests, the following workflow can
+be used:
+
+1. Install `minikube` and `helm`.
+2. Start a `minikube` cluster via `minikube start`.
+3. From the `gitea/helm-chart` directory execute the following command. This
+   will install the dependencies listed in `Chart.yml` and deploy the current
+   state of the helm chart found locally. If you want to test a branch, make
+   sure to switch to the respective branch first.
+  `helm install --dependency-update gitea . -f values.yaml`.
+4. Gitea is now deployed in `minikube`. To access it, it's port needs to be
+   forwarded first from `minikube` to localhost first via `kubectl --namespace
+   default port-forward svc/gitea-http 3000:3000`. Now Gitea is accessible at
+   [http://localhost:3000](http://localhost:3000).