More criteria and questions

[dtolnay]

I moved this brainstorming list from the bottom of README.md. I will remove entries as I file tickets for them or incorporate them into guidelines.

• Don't overpromise
• Let clients choose what to throw away
• Tier-X platform support
• Version number
• README.md
• Ecosystem interop

[dtolnay]

I don't think the remaining entries need guidelines.