proposal: streamline process description in README
proposal: streamline process description in README

The old doc takes too long to get to explaining the process,
and it spends too much text on it. There's no "at a glance"
explanation of the process. Add one.

Also explain the proposal review meetings.

Change-Id: Ib74cff06904f990dd8abcb4c3cc1097d7656f3b5
Reviewed-on: https://go-review.googlesource.com/136515
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>

golang/proposal,golang/proposal