docs: add a section on getting the client working with celery

docs/celery.rst

@@ -0,0 +1,19 @@
+****************************************
+Running with Celery
+****************************************
+
+To use this client with Celery, call the `initialize_client` method in the `worker_process_init` event. If you don't initialize the client inside the `worker_process_init` event, the client will not poll for updates. As such, the client will not update its feature toggles, even when changes are made server side.
+
+.. code-block:: python
+
+    from UnleashClient import UnleashClient
+    from celery.signals import worker_process_init
+
+    client = UnleashClient(
+        url="http://localhost:4242/api/",
+        app_name="test-dev",
+        custom_headers={'Authorization': '*:development.bb09e81624d5ad67b2ac29bd0b0fdc35ccbac884e63cfd20c6fefc49'})
+
+    @worker_process_init.connect
+    def configure_workers(sender=None, conf=None, **kwargs):
+        client.initialize_client()

docs/index.rst

@@ -23,6 +23,7 @@ Documentation for Unleash's Python client.  See the sidebar for more topics!
     customcache
     development
     wsgi
+    celery
 
 .. toctree::
     :caption: API Documentation