Add last updated time

README.md

                 /{network2}/...
 ```
 
+There is also a special node at `/docker/_updated` which is updated with
+the current unix timestamp after an update has finished. This may be useful
+to watch in scripts that need to trigger after container changes (listening
+to the containers would trigger as soon as the first child node is written,
+so is probably not useful in most situations).
+
 ## Current known issues
 
 * The docker node is deleted when the script starts, so you can't run multiple
   copies on multiple hosts
-* There's no way to get notified when the script has finished, rather than
-  mid-update
 

updater.py

 #!/usr/bin/env python3
 
 import etcd
+import time
 
 
 class Updater:
       for k, v in container['net']['addr'].items():
         self._write('/networks/%s/%s' % (k, name), v)
       self._write('/hosts/%s/%s' % (container['host'], name), name)
+    self._notify_update()
 
 
   def remove_containers(self, old_containers):
       for k, v in container['net']['addr'].items():
         self._delete('/networks/%s/%s' % (k, name)) 
       self._delete('/hosts/%s/%s' % (container['host'], name))
+    self._notify_update()
+
+  def _notify_update(self):
+    self._write('/_updated', time.time())