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.
While we're updating this, it may be worth clarifying if the author of the RFC is expected to implement the feature or not. Maybe there should at least be some understanding of who's going to implement it and when? There are some accepted RFCs that have just been sitting there without an implementation for a while now.