Add list of contributors to each release

[angela97lin]

trafficstars

As we expand into the OS community and outside users contribute to EvalML, it would be nice to have a list of contributors for each release as Featuretools, Woodwork, and other libraries have, to recognize those contributors.

We do release really often though, so that's something to consider 🤔

https://woodwork.alteryx.com/en/stable/release_notes.html https://featuretools.alteryx.com/en/stable/release_notes.html

[dsherry]

Great!

This issue tracks updating our release.md to document the additional process during release.

Extra credit if you can update tools/format_release_notes.sh to automatically figure out who the contributors were since last release!