Add CONTRIBUTING.md

[lsetiawan]

Overview

This is a duplicate of uw-ssec/python-project-template#4.

Need to add CONTRIBUTING.md with instructions on how to contribute to the open source software. For some text sample and guidelines, refer to the links below:

• https://www.dataschool.io/how-to-contribute-on-github/
• https://github.com/github/docs/blob/main/CONTRIBUTING.md