Clean up README for the first ytsm release.

Because ytsm has been operating so smoothly lately, I'm going to tag a
release sometime in the near future, probably either today or tomorrow.
This will give me more of a reason to try out some major changes I've
been thinking about for a while.

Anyway, nothing exciting in this commit. A few version numbers were
updated, a question about releases was added, some performance notes
from six months ago were removed because they made a big deal about
nothing, and a couple other minor changes.