svn-fe: warn about experimental status

svn-fe is young and some coming cleanups might involve backward
incompatible UI changes.  Add some words of warning to the manual so
early adopters that are not following the project closely don't get
burned.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/svn-fe/svn-fe.txt

@@ -18,6 +18,9 @@ Subversion repository mirrored on the local disk. Remote Subversion
 repositories can be mirrored on local disk using the `svnsync`
 command.
 
+Note: this tool is very young.  The details of its commandline
+interface may change in backward incompatible ways.
+
 INPUT FORMAT
 ------------
 Subversion's repository dump format is documented in full in