Document allow/disallow feature

README.rst

@@ -51,6 +51,23 @@ collections in your settings::
 If you do not use `django-yarnpkg`_, construct the path manually, ot use
 whatever mechanism your asset manager provides.
 
+You can configure which icon collections are available using two settings:
+
+  ICONIFY_COLLECTIONS_ALLOWED = ["foo", "bar"]
+
+This list controls which collections can be used. If it is set to a
+non-empty list, only the collections listed are allowed.
+
+  ICONIFY_COLLECTIONS_DISALLOWED = ["foo", "bar"]
+
+This list, on the other hand, controls which collections cannot be used.
+If this list contains values, while `COLLECTIONS_ALLOWED` doesn't, all
+collections except the listed ones are allowed.
+
+The allow/disallow feature can be used in cases where only a limited set
+of collections should be available due to design principles or for legal
+reasons.
+
 Finally, include the URLs in your `urlpatterns`::
 
   from django.urls import include, path