docs: Add comments linking to new events doc.

zerver/lib/events.py

@@ -1,3 +1,5 @@
+# See http://zulip.readthedocs.io/en/latest/events-system.html for
+# high-level documentation on how this system works.
 from __future__ import absolute_import
 from __future__ import print_function
 

zerver/tests/test_events.py

@@ -1,4 +1,6 @@
 # -*- coding: utf-8 -*-
+# See http://zulip.readthedocs.io/en/latest/events-system.html for
+# high-level documentation on how this system works.
 from __future__ import absolute_import
 from typing import Any, Callable, Optional
 

zerver/tornado/event_queue.py

@@ -1,3 +1,5 @@
+# See http://zulip.readthedocs.io/en/latest/events-system.html for
+# high-level documentation on how this system works.
 from __future__ import absolute_import
 from typing import cast, AbstractSet, Any, Optional, Iterable, Sequence, Mapping, MutableMapping, Callable, Union, Text