[cmucl-imp] Upcoming release

[Raymond Toy]

I noticed that it has been a year since our last release, so it's time
once again for another release.  A new branch has been created
(RELEASE-20c-BRANCH) for the release.

This time, I'm not going to do any pre-release.  We'll make just the one
build due at the end of this month for the 20c release.  If it's broken
for some reason, we'll send a patch if possible.  If it's really broken,
we'll recommend people to use the following snapshot.  (Of course,
because of the release, there will not be a snapshot for Nov.)

Ray