Add an introduction to the federation documentation discussing
background information on identity federation and how it is implemented
in keystone.
This repurposes some of the content in this blog post[1] of which I am
the author.
[1] http://www.gazlene.net/demystifying-keystone-federation.html
Partial-bug: #1793374
Change-Id: I5f3a5e70c7b868762880930ea6277691f44c046a