2.21.0 and announcements

[David Kastrup]

So it's time to roll out 2.21.0 and I think the build instructions are
ok-ish for that.  We don't clarify the relations/recommendations for
Guile-1.8/2.x but that relation/recommendation is currently in flux and
this is an unstable release.

We still haven't made the "big" 2.20 announcement, and 2.21.0 being out
would make the web page consistent with regard to stable/unstable, so
that would seem like a good opportunity.

Except that the web page currently is broken with some Python2/3
problems, and that people are hitting a roadblock with MacOSX.  I think
we should point out on our download page that the GUB-provided
installers are _only_ for 32bit MacOSX, and that separate downloads may
be provided elsewhere (possibly linking to Marnen's page in the hope
that it does not get overloaded?).

Because once the big announcements are made, we want people to actually
be able to work with what they find on the web page.

We can release 2.21.0 prior to that, of course, since the web page
updates are (I think) independent from releases.

Thoughts?

-- 
David Kastrup