Add contributor guidelines

[hareball90]

Contributor guidelines should include:

• PR template
• stating what acknowledgement/authorship a contributor is entitled to

[hareball90]

by contributing, acknowledge that they are assigning copyright to us so we can continue to maintain package, always be licensed under gpl2

reviewer to add contributor to readme

[hareball90]

This all-contrbutors looks like it could help? https://github.com/all-contributors/all-contributors

[twomagpi]

Partially resolved by #76

[twomagpi]

I think all-contributors is probably to much technology to solve the problem of adding people to the README. Can revisit if the project becomes bigger.