[AD] 5.0.0 RC2 release notes

[Peter Wang <novalazy@xxxxxxxxxx>]

Sorry about the flood of messages on the alleg-commits list.

Merging the 5.1 changes into the 5.0 branch on my local machine went as
well as I hoped.  I was able to cherry-pick the changes made in 5.1,
excepting those commits which were ultimately discarded (e.g. the
initial attempt to deal with al_get_opengl_version).  If there had been
experimental changes that needed to be discarded, I could have done it.

Pushing the merged branch back to SVN wasn't so great.  At usual, the
server is very slow so 60-odd commits took quite a while (the email
timestamps will tell you).  The flood of emails is annoying a few of you
right now, no doubt.

I couldn't find a way to retain the original author of each patch, so
everything is credited to me (your visualisations are ruined).  The next
best thing seems to be allowing git-svn to append those From: lines,
which will be taken into account if we ever convert to a git repository
proper.

Peter