Document entrypoint (#215)

Missed in #187.

README.rst

@@ -121,3 +121,21 @@ altogether, you can use the ``-p`` argument, for example:
 .. code-block:: sh
 
     pytest -p no:randomly
+
+Entry Point
+-----------
+
+If you're using a different randomness generator in your third party package,
+you can register an entrypoint to be called every time ``pytest-randomly``
+reseeds. Implement the entrypoint ``pytest_randomly.random_seeder``, referring
+to a function/callable that takes one argument, the new seed (int).
+
+For example in your ``setup.cfg``:
+
+.. code-block:: sh
+
+    [options.entry_points]
+    pytest_randomly.random_seeder =
+        mypackage = mypackage.reseed
+
+Then implement ``reseed(new_seed)``.