cvsimport: update documentation to include separate remotes option

Document the cvsimport -r <remote> option which switches cvsimport
to using a separate remote for tracking branches.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-cvsimport.perl

@@ -40,7 +40,7 @@ Usage: ${\basename $0}     # fetch/update GIT from CVS
        [-o branch-for-HEAD] [-h] [-v] [-d CVSROOT] [-A author-conv-file]
        [-p opts-for-cvsps] [-P file] [-C GIT_repository] [-z fuzz] [-i] [-k]
        [-u] [-s subst] [-a] [-m] [-M regex] [-S regex] [-L commitlimit]
-       [CVS_module]
+       [-r remote] [CVS_module]
 END
 	exit(1);
 }