Ensure only one object can be created chapter

docs/only_one.rst

@@ -1,2 +1,16 @@
 How to allow creating only one object from the admin?
 =====================================================
+
+The UMSRA admin has asked you to limit the number of categories to only one. They want every entity to be of the same category.
+
+You can do this by::
+
+    MAX_OBJECTS = 1
+
+    def has_add_permission(self, request):
+        if self.model.objects.count() >= MAX_OBJECTS:
+            return False
+        return super().has_add_permission(request)
+
+This would hide the add buton as soon as one object is created. You can set :code:`MAX_OBJECTS` to any value to ensure that
+larger number of objects can't be ceated than :code:`MAX_OBJECTS`.