Start on invitation backends documentation

docs/usage.rst

@@ -1,9 +1,13 @@
+===========
 Basic usage
 ===========
 
 After installing django-organizations you can make basic use of the accounts
 with minimal configuration.
 
+Views
+=====
+
 The application's default views and URL configuration provide functionality for
 account creation, user registration, and account management.
 
@@ -50,3 +54,13 @@ Changing ownership
 Changing ownership of an organization is as simple as updating the
 `OrganizationOwner` such that it points to the new user. There is as of yet no
 out of the box view to do this, but adding your own will be trivial.
+
+Invitation & registration backends
+==================================
+
+The invitation and registration backends provide a way for your account users
+to add new users to their accounts and if your application allows it, for users
+to create their own accounts at registration. Each base backend class is
+designed to provide a common interface which your backend classes can use to
+work with whatever user models, registration systems, additional account
+systems, or any other tools you need for your site.