Add Contributions guidelines

I found a tiny issue in the repo and put up a PR to address: #48

As I was doing so, it wasn't clear to me if there were any guidelines the maintainers wanted me to follow. Even a simple ("feel free to open a PR") would be better than nothing.

GH recommendations for this kinda thing if you wanna put something a little more standardized down: https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors