Add documentation for the changes introduced by "Implement and Enable
Community Images", I94bc7708b291ce37319539e27b3e88c9a17e1a9f. Also
added some text to clarify the implications of the owner_is_tenant
configuration option when identifying an image producer or consumer.
Added an introductory section about the kinds of sharing available.
Change-Id: I58f39d4729e783a86999b1fe18debd97a3ebe544
fbdd3381ca