Feature: Add CONTRIBUTING.md

[bobbyiliev]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like

Add a CONTRIBUTING.md file:

# Contributor instructions

## Testing

???

## Cutting a new release

Perform a manual test of the latest code on `main`. See prior section. Then run:

    git tag -a vX.Y.Z -m vX.Y.Z
    git push origin vX.Y.Z