obsolete section in README

[joachim-n]

This bit looks like it's no longer the case on D8:

Notice how the relation of the user to the group also includes the OG audience field name this association was done by. Like this we are able to express different membership types such as the default membership that comes out of the box, or a "premium membership" that can be for example expired after a certain amount of time (the logic for the expired membership in the example is out of the scope of OG core).

Having this field separation is what allows having multiple OG audience fields attached to the user, where each group they are associated with may be a result of different membership types.

The Og::createMembership() doesn't take an audience field name.

[reinette]

Looking at the OG UI implementation, it seems that large chunks of the README are obsolete for D8 - there is no permissions or role management.

[zerolab]

Permissions/roles is provided by #243 -- please test and feed back on it.

+1 on cleaning up documentation