docs: initial contribution guide (#531)

CONTRIBUTING.md

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+# Contributing
+
+[asdf](https://asdf-vm.com) is used to manage the development tools for these
+GitHub Actions. See it's documentation to install and setup on your machine.
+
+## Setup
+
+```shell
+# add plugins with asdf
+asdf plugin add nodejs
+asdf plugin add yarn
+asdf plugin add lefthook
+# install tools
+asdf install
+# install nodejs deps & setup lefthook
+yarn install
+```
+
+## GitHub Actions
+
+Contributions are welcome. Please raise an issue to discuss changes before
+raising a Pull Request.
+
+Source code for the Actions can be found in the `lib/` directory.
+
+## Code Quality
+
+[Lefthook](https://github.com/evilmartians/lefthook) is used to run `pre-commit`
+and `pre-push` git hooks to ensure standardised best practices are followed
+before raising code changes in Pull Requests. It will be installed and
+configured by the above steps.