Add dot format to the usage text

topydo/commands/ListCommand.py

@@ -130,6 +130,8 @@ When an expression is given, only the todos matching that expression are shown.
 -f : Specify the output format, being 'text' (default), 'ical' or 'json'.
 
      * 'text' - Text output with colors and indentation if applicable.
+     * 'dot'  - Prints a dependency graph for the selected items in GraphViz
+                Dot format.
      * 'ical' - iCalendar (RFC 2445). Is not supported in Python 3.2. Be aware
                 that this is not a read-only operation, todo items may obtain
                 an 'ical' tag with a unique ID. Completed todo items may be