Please annotate Overview in timeline.md 

http://jupyterhub-tutorial.readthedocs.io/en/latest/timeline.html#overview-of-jupyterhub

Please annotate these subsections:

<img width="666" alt="screen shot 2016-10-21 at 05 49 45" src="https://cloud.githubusercontent.com/assets/2680980/19596075/72187d1c-9752-11e6-9937-7fc92083c922.png">
