Add contributing guide (#298)

CONTRIBUTING.md

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+# Contributing Guidelines
+
+Contributions are welcome via GitHub Pull Requests. This document outlines the process to help get your contribution accepted.
+
+Any type of contribution is welcome; from new features, bug fixes, tests, documentation improvements, or even adding charts to the repository.
+
+## How to Contribute
+
+1. Fork this repository, develop, and test your changes.
+2. Submit a pull request.
+
+***NOTE***: To make the Pull Requests' (PRs) testing and merging process easier, please submit changes to multiple charts in separate PRs.
+
+Remember to always work in a branch of your local copy, as you might otherwise have to contend with conflicts in the main branch.
+
+### Technical Requirements
+
+When submitting a PR make sure that:
+
+- You accepted the [CLA](https://colineberhardt.github.io/cla-bot/#what-is-a-cla);
+- Your PR passes CI jobs for linting and tests (automatically done by the CI pipeline);
+- Your PR follows [Helm best practices](https://helm.sh/docs/chart_best_practices/);
+- Any change (even if it only updates the docs) to a chart leads to a version bump following [semver](https://semver.org/) principles. This is the version that is going to be merged in the GitHub repository and published to the Helm registry. Update the version in the `Chart.yaml` file.
+
+### Documentation Requirements
+
+- Make sure to properly annotate each parameter you add/update in the `values.yaml` file. The table of parameters is generated based on these annotations from the `values.yaml` file using [helm-docs](https://github.com/norwoodj/helm-docs);
+- Do NOT edit `README.md` directly. Instead make changes to `README.md.gotmpl`. `README.md` will be auto-generated as part of pre-commit hooks.
+
+### Pre-commit Hooks
+This repository is using `pre-commit` hooks which are executed using [pre-commit](https://pre-commit.com/). Pre-commit hooks allow to execute scripts on the local developer's machine before committing the changes to the repository. We're using `pre-commit` hooks to auto-generate `REAMDE.md` based on the templates. For example, check out [Node Helm chart README.md](charts/node/README.md.gotmpl)
+
+Follow the [instructions](https://pre-commit.com/#install) to install `pre-commit`. `REAMDE.md` will be updating automatically after that.
+
+To manually render `README.md`, you should install [`helm-docs`](https://github.com/norwoodj/helm-docs), navigate to repository root and run:
+```
+helm-docs --chart-search-root=charts/<CHART_NAME> --template-files=README.md.gotmpl
+```
+
+You may encounter `files were modified by this hook` error after updating `README.md.gotmpl` file when using `pre-commit`.
+This is intended behaviour. Make sure to run `git add -A` once again to stage changes in the auto-updated `REAMDE.md`