set up automatic contributor updates

[dmarticus]

In the CONTRIBUTING.md section of this project we have a line that says

You will be automatically included in the AUTHORS.md file as contributor in the next release. If you encounter any confusion or frustration during the contribution process, please create a GitHub issue and we'll do our best to improve the process.

but it doesn't look like that process is working, since the AUTHORS.md file doesn't list any authors at all.