Update the contribution guidelines

[cromedome]

This already covered a lot of ground, and now it covers even more:

• Itemize new ways to contribute
• Reminds people to run tests
• Reasons we may request changes to/reject pull requests

A few revisions to language throughout, and in preparation of other upcoming changes, replaced master with main, and fixed some nagging http: links.

[cromedome]

Merged. Thanks for the feedback!