Adding Documentation for Enabling External Authentication - Depends on External auth3 PR# 32

Adding ManageIQ/Guides for installing and enabling
external authentication via IPA.

Depends on the External auth3 Pull Request https://github.com/ManageIQ/manageiq/pull/32


I'd change the sentence above to:
This document identifies the RPM packages to install on an
Appliance to be able to use External Authentication (httpd).