[WIP] Adding Documentation for Enabling External Authentication - Depends on External auth3 pull request.

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


Maybe just httpd?

The thought is we want it to be known who created this file.
Maybe a comment in the top would be better?

@abellotti Think this adds the ability for anyone with pam to login into the appliance, not just httpd. confirm?