README.md: update proposal steps
README.md: update proposal steps

Update the proposal steps based on my feedback from the proposal process.

Change-Id: I27e4e1a4cbad123b3fe6fe06a62065cf4da94b91
Reviewed-on: https://go-review.googlesource.com/14231
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/proposal,golang/proposal