design: release proposal draft three
design: release proposal draft three

Updates golang/go#12302

This update includes:

 - Make it clear that as SemVer requires versions to be immutable
   tags must also be immutable.
 - Avoid possibly confusion between a gb project and the use of
   the word within this proposal by removing most uses of 'project'.
 - Draft proposal for deprecating the previous advice to encode a
   version number in a package's import path.

Change-Id: I395b872a673f537930ecf6f10631b82a0a0bcaf5
Reviewed-on: https://go-review.googlesource.com/14415
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/proposal,golang/proposal