Update CONTRIBUTING.md

[bccummings]

I should make a contributing file.

Some good guidelines:

https://help.github.com/en/articles/setting-guidelines-for-repository-contributors
https://gist.github.com/PurpleBooth/b24679402957c63ec426

[bccummings]

In the future, should add more technical instructions on how to clone the repository, make a branch/fork & pull request, etc.

Also add more detail concerning the issue tracker.