Added some registration notes to start the docs

docs/registering_models.rst

@@ -0,0 +1,24 @@
+==================
+Registering Models
+==================
+
+
+
+
+You can register models with the 
+
+import categories
+categories.register_fk(MyModel)
+
+or
+
+import categories
+categories.register_m2m(MyModel)
+
+If you want more than one field on a model you have to have some extra arguments
+
+import categories
+categories.register_fk(MyModel, 'primary_category')
+categories.register_fk(MyModel, 'secondary_category', {'related_name':'mymodel_sec_set'})
+
+The ``extra_args`` allows you to specify the related_name of one of the fields so it doesn't clash.