Contributing docs

There's anecdotally a number of people around that would like to help out on this repo, but don't know how to get setup / started working on it.

Could you please a add a `CONTRIBUTING` file [reference](https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors) with a step by step how to setup a local setup for developing this plugin?

Start with the linked GitHub contributor-guidelines reference and inspect the repository to identify the local setup steps needed for this WordPress plugin. Add a CONTRIBUTING file with a step-by-step development setup guide; it is done when a new contributor can follow the instructions to prepare a local environment.

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