Add 'CONTRIBUTING.md' File

[og-mrk]

Moved two sections from README.md file into the new CONTRIBUTING.md file, condensing the length of README a little bit.

[og-mrk]

@ChrisTitusTech if you want to edit the wording & improve it even more, you can do so by doing Edit File on either files (I've enabled Allow edits and access to secrets by maintainers).