Add missing docstring

Signed-off-by: Chris Warrick <kwpolska@gmail.com>

nikola/plugins/command/auto/__init__.py

@@ -281,6 +281,7 @@ def run_nikola_build(self, event):
 
     @asyncio.coroutine
     def run_rebuild_queue(self):
+        """Run rebuilds from a queue (Nikola can only build in a single instance)."""
         while True:
             date, event_path = yield from self.rebuild_queue.get()
             if date is None: