release: start subcommand takes an optional base as well

Signed-off-by: Stefan Naewe <stefan.naewe@googlemail.com>

git-flow-release

@@ -44,7 +44,7 @@ PREFIX=$(git config --get gitflow.prefix.release)
 
 usage() {
 	echo "usage: git flow release [list] [-v]"
-	echo "       git flow release start [-F] <version>"
+	echo "       git flow release start [-F] <version> [<base>]"
 	echo "       git flow release finish [-Fsumpk] <version>"
 	echo "       git flow release publish <name>"
 	echo "       git flow release track <name>"