Add readme note for async_callbacks

Author: altendky

README.rst

@@ -61,6 +61,19 @@ functions, which take funcargs, does not work. Please use
       res = yield threads.deferToThread(os.listdir, tmpdir.strpath)
       assert res == []
 
+
+async_callbacks
+=================
+Using `twisted.internet.defer.ensureDeferred` as a decorator for test
+functions, which take funcargs, does not work. Please use
+`pytest_twisted.async_callbacks` instead::
+
+  @pytest_twisted.async_callbacks
+  async def test_some_stuff(tmpdir):
+      res = await threads.deferToThread(os.listdir, tmpdir.strpath)
+      assert res == []
+
+
 Waiting for deferreds in fixtures
 =================================
 `pytest_twisted.blockon` allows fixtures to wait for deferreds::