Add contributor guidelines

[brianhuffman]

Github has special support for displaying/linking to contributor guidelines when anyone creates a pull request.
https://docs.github.com/en/github/building-a-strong-community/setting-guidelines-for-repository-contributors

We should create a contributing.md file for the saw-script repo so we can take advantage of this. This will become ever more important as the number of active contributors to saw-script grows.

[brianhuffman]

In particular, the contributor guidelines should include a description of how to use the Mergify bot.