Make separate article for access control using multiple user models

[crandmck]

http://loopback.io/doc/en/lb3/Authentication-authorization-and-permissions.html#preparing-access-control-models

Pull into a separate article.

n.b. strongloop/loopback#3231 (comment)

NOTE from bajtos:

I setup http://ibm.biz/setup-loopback-auth shortcut to point to http://loopback.io/doc/en/lb3/Authentication-authorization-and-permissions.html#preparing-access-control-models. If/when you move the setup instructions to a different page, then we need to change the target url of the shortcut http://ibm.biz/setup-loopback-auth points to.

[dhmlau]

Closing due to inactivity.