Add section "Acknowledging the use of Enrico" to docs

We should add a section "Acknowledging the use of Enrico" to our top-level docs page [here](http://enrico.readthedocs.org/en/latest/), similar to what http://www.astropy.org did, and maintain a list of papers that used Enrico.

@davidsanchez Is this the final link or is there an ICRC (non-arXiv) link?
http://adsabs.harvard.edu/abs/2013arXiv1307.4534S

We should also link to this tutorial:
https://fermi-hero.readthedocs.org/en/latest/index.html