patman: document default 'send' command

Document that this command is the default and what it's intended for.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Maxim Cournoyer <maxim.cournoyer@savoirfairelinux.com>
diff --git a/tools/patman/__main__.py b/tools/patman/__main__.py
index 70968f6..ece59c5 100755
--- a/tools/patman/__main__.py
+++ b/tools/patman/__main__.py
@@ -55,7 +55,8 @@
                     default=False, help='Display the README file')
 
 subparsers = parser.add_subparsers(dest='cmd')
-send = subparsers.add_parser('send')
+send = subparsers.add_parser(
+    'send', help='Format, check and email patches (default command)')
 send.add_argument('-i', '--ignore-errors', action='store_true',
        dest='ignore_errors', default=False,
        help='Send patches email even if patch errors are found')