Fix #12 - defining a tenant does not define an association index, and manually doing so causes error (pranav-g10)

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Fix #11 - Allow for index without tenant_field

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>
Update the documentation

Signed-off-by: Aymeric Brisse <aymeric.brisse@perfect-memory.com>