Add CONTRIBUTING.md about mailman

[skosch]

Agreed, always nice to have. Personally I think as long as tests pass and the code is reasonably sane, PRs are very welcome, bonus points for new tests and docs and @specs. And if someone wanted to commit (ha!) more time I wouldn't hesitate giving them push access. @arunvelsriram Do you know of any project that has a good guide we can steal?

@kamilc, @Ch4s3, any thoughts?

[arunvelsriram]

@skosch Off the top of my head I would say Terraform and Nevergreen have a good contribution guide. Terraform's guide is very extensive so we can steal bits and pieces from it.

I have seen other good guides as well but don't remember the project names.