Section on SNI

At @seanherron's [suggestion](https://github.com/18F/api-standards/pull/27#issuecomment-47024698), I added a section on [Server Name Indication](http://en.wikipedia.org/wiki/Server_Name_Indication). This is particularly meaningful after the experience of attempting SNI support for [openFDA](https://open.fda.gov/).

@seanherron, feel free to merge this yourself if it merits your :+1:, or edit it in-place on the `sni` branch.