Documentation: LP#1386854 - Locally Hosted Added Content.

Adds documentation that describes how to use the local content bypass
feature of the AddedContent modules.  This allows for local added content like
cover art, reviews, TOC, exerpts or annotations to be shown in bib records.

I also changed the headers in the External Added Content section to use title case.

Signed-off-by: Josh Stompro <stomproj@larl.org>
Signed-off-by: Yamil Suarez <yamil@yamil.com>