Allow the extension for adding todo notes to rst files. Add information on how to use this extension to the contribution section.

docs/conf_common.py

@@ -154,8 +154,14 @@ extensions = ['breathe',
               'sphinxcontrib.rackdiag',
               'sphinxcontrib.packetdiag',
               'html_redirects',
+              'sphinx.ext.todo',
               ]
 
+# sphinx.ext.todo extension parameters
+# If the below parameter is True, the extension
+# produces output, else it produces nothing.
+todo_include_todos = False
+
 # Enabling this fixes cropping of blockdiag edge labels
 seqdiag_antialias = True