Add Contributing, Pull Request, and Code of Conduct files

[bojand]

See GitHub docs:

https://help.github.com/en/github/building-a-strong-community/setting-guidelines-for-repository-contributors https://opensource.guide/starting-a-project/#writing-your-contributing-guidelines https://opensource.guide/starting-a-project/#establishing-a-code-of-conduct

This should be added to the website as well.