Add Docset Feed, expand Install section in README

The docset feed will help with having updates to the XEP docset be
pulled in automatically by applications, and streamlines installation
thereof.

Initial commit of XMPP XEP Dash Docset Builder

This includes a fairly complete set of commands for building XMPP XEPs
as a Dash Docset, useful in various environments (e.g. Emacs, Vim, etc.)
and based on the source files themselves.