docs: add contribution doc

CONTRIBUTING.md

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+# Development
+
+## Contributing
+
+Code contributions are done through [GitHub Pull requests](https://github.com/grafana/grafana-crossplane-libsonnet/pulls), each pull request requires CI to pass and at least one review. We follow [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/), pull requests will generally be merged with a squash merge.
+
+> **Hint**: Because the generation process changes a lot of code, it is highly encouraged to put the generated code into a separate commit to make reviewing easier.
+>
+> If the branch diverges from `main`, then do a rebase and drop the commit with the generated code, followed by regenerating everything into a new commit.
+
+Bugs or feature requests can go into [GitHub Issues](https://github.com/grafana/grafana-crossplane-libsonnet/issues), other questions can be asked through [GitHub Discussions](https://github.com/grafana/grafana-crossplane-libsonnet/discussions).
+
+## Generation process
+
+`make build` will generate the libraries and packages, including the docs in `docs/`.
+
+## Directory layout
+
+`generator/` is where the code generator lives.
+
+`packages/` and `docs/` are completely generated, do not edit these, changes will be overwritten by the generation process.
+
+`grafanaplane/` except for `grafanaplane/main.libsonnet` and `grafanaplane/example.jsonnet` are generated as well.