Create a CONTRIBUTING.md file with guidelines for contributors

[LegalizeAdulthood]

See the github documentation here:

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

In particular I was unaware of the convention for commit messages to start with a label identifying the area of change, e.g. PDP11: ....

[rcornwell]

This helps when scanning commit logs to see which commits are important to your problem, and probably are not applicable.