Clarify the m2m intermediary docs a bit.

      on the source model (this would be ``Group`` in our example). If you

      have more than one foreign key, a validation error will be raised.

    * If the many-to-many relation is a relation on itself, the relationship

      must be non-symmetric.

    * The only exception to this is a model which has a many-to-many

      relationship to itself, through an intermediary model. In this

      case, two foreign keys to the same model are permitted, but they

      will be treated as the two (different) sides of the many-to-many

      relation.

    * When defining a many-to-many relationship from a model to

      itself, using an intermediary model, you *must* use

      ``symmetrical=False`` (see the documentation for

      ``ManyToManyField`` above).

Now that you have set up your ``ManyToManyField`` to use your intermediary 

model (Membership, in this case), you're ready to start creating some