Added storage default help function

plugins/storage/functions

@@ -2,6 +2,25 @@
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
 
+help_formatted (){
+	echo -e 'Usage: dokku storage:COMMAND'
+    echo ''
+    echo 'Mount local volume / directories inside containers.'
+    echo ''
+    echo 'Additional commands:'
+    help_content_func | sort | column -c2 -t -s,
+}
+
+help_content_func () {
+      declare desc="return storage plugin help content"
+      cat<<help_content
+    storage, Mount local volume / directories inside containers
+    storage:list <app>, List bind mounts for app's container(s) (host:container)
+    storage:mount <app> <host-dir:container-dir>, Create a new bind mount
+    storage:unmount <app> <host-dir:container-dir>, Remove an existing bind mount
+help_content
+    }
+
 verify_paths() {
   declare desc="verifies storage paths"
   echo "$1" | grep -qe '^/.*\:/' || dokku_log_fail "Storage path must be two valid paths divided by colon."