[Svnmerge] Document SVN requisites

[Giovanni Bajo]

Hello,

I've been just informed that svnmerge.py does not say anything about which SVN
client/server version it supports. Though we don't have a formal documentation
(man-page or whatever) for it, I think this useful piece of information should
be at least mentioned in the top comment. I personally have been using it on
SVN server/client 1.1, but this was some months ago, when I was still the only
one developing the Python version.

I'm going to add a comment saying that it requires SVN server/client 1.1, and
it's never been tested on anything older. Speak up if you have experiences to
share.

Thanks
Giovanni Bajo