git-svn: add a message encouraging use of SVN::* libraries

I'm using svn 1.4.0-4 in Debian unstable and apparently there's
a regression on the SVN side that prevents a symlink from
becoming a regular file (which git supports, of course).

It's not a noticeable regression for most people, but this broke
the full-svn-tests target in t/Makefile for me.

The SVN::* Perl libraries seem to have matured and improved over
the past year, and git-svn has supported them for several months
now, so with that I encourage all users to start using the
SVN::* Perl libraries with git-svn.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>