Outline Minds Bounties program

After some discussions with @ottman I'd like to propose an initial outline for the Minds Bounty program by adding what we've come up with so far to the docs as a Work-In-Progress, Request-For-Feedback sort of way.

I'll be adding information we've come up with so far and cleaning up the related pages/links where necessary, such as the Ways to Contribute section.

• Add page about Minds Bounty Program under Contributing section
• Clean up Contributing page where necessary to reflect Minds Bounty Program