release: start subcommand takes an optional base as well

Signed-off-by: Stefan Naewe <stefan.naewe@googlemail.com>
diff --git a/git-flow-release b/git-flow-release
index 05815bc..4572d81 100644
--- a/git-flow-release
+++ b/git-flow-release
@@ -44,7 +44,7 @@
 
 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>"