Give pre-RFC suggestions and clarify PR process

Based on workweek discussions here are some minor updates to the RFC process.

TL;DR is that we are no longer going to do RFC approvals at the meeting, are going to try to have more ad-hoc meetings involving a wider group of contributors, will insist on summaries of those meetings being delivered back to the PR threads, core team will make final decisions based on discussion in the thread.
We will try to invite community stakeholders to more weekly meetings to discuss their RFCs as needed. I've also added some text about how to properly prepare *before* submitting an RFC.
Added some text about this.