DOC add phoenix channel transport example (#354)

Author: leszekhanusz

docs/code_examples/phoenix_channel_async.py

@@ -0,0 +1,30 @@
+import asyncio
+
+from gql import Client, gql
+from gql.transport.phoenix_channel_websockets import PhoenixChannelWebsocketsTransport
+
+
+async def main():
+
+    transport = PhoenixChannelWebsocketsTransport(
+        channel_name="YOUR_CHANNEL", url="wss://YOUR_URL/graphql"
+    )
+
+    # Using `async with` on the client will start a connection on the transport
+    # and provide a `session` variable to execute queries on this connection
+    async with Client(transport=transport) as session:
+
+        # Execute single query
+        query = gql(
+            """
+            query yourQuery {
+                ...
+            }
+        """
+        )
+
+        result = await session.execute(query)
+        print(result)
+
+
+asyncio.run(main())

docs/transports/phoenix.rst

@@ -10,6 +10,8 @@ framework `channels`_.
 Reference:
 :class:`gql.transport.phoenix_channel_websockets.PhoenixChannelWebsocketsTransport`
 
+.. literalinclude:: ../code_examples/phoenix_channel_async.py
+
 .. _Absinthe: http://absinthe-graphql.org
 .. _Phoenix: https://www.phoenixframework.org
 .. _channels: https://hexdocs.pm/phoenix/Phoenix.Channel.html#content