Add CONTRIBUTING.md to document contribution workflow

[ryneeverett]

https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors

A good starting point is probably this comment:

OK - the workflow we have is

1. If there is a problem, create an issue describing the problem, and explaining how to reproduce it

2. Make the code changes necessary to fix the problem

3. Test the changes

4. Submit a PR including
   
   1. the commits implementing the changes
   2. description and / or documentation of how the changes were tested

[petefoth]

Thanks for this. We a have a number of outstanding document issues, and we don't currently have the bandwidth to do much about them. The project's maintainers' available time is mostly spent keeping the monthly production of builds for ~250 devices ticking over..

We are very happy to accept PRs, particularly to address specific issues ;)