adding example for custom oauth2 storage to readme

README.md

@@ -165,6 +165,41 @@ if __name__ == "__main__":
     print(json.dumps(a.get_pipelines()))
 ```
 
+# Advanced oauth2 token storage - thanks [@sangaline](https://github.com/sangaline)\!
+
+This is an example of how you can use the `oauth2_token_getter` and `oauth2_token_setter` kwargs on the client to use custom storage (in this case redis) so that multiple clients can share the same access/refresh tokens generated by the oauth2 requests.
+
+```python
+import aioredis
+from hubspot3 import Hubspot3
+
+
+redis_client = await aioredis.create_redis_pool(url, db=db, encoding='utf-8', timeout=10)
+
+def oauth2_token_getter(token_type: str, client_id: str) -> str:
+    loop = asyncio.get_event_loop()
+    key = f'hubspot-oauth2-tokens:{token_type}:{client_id}'
+    return loop.run_until_complete(redis_client.get(key))
+
+def oauth2_token_setter(token_type: str, client_id: str, token: str) -> None:
+    loop = asyncio.get_event_loop()
+    key = f'hubspot-oauth2-tokens:{token_type}:{client_id}'
+    # Token expiration is six hours, so match that when we store the tokens.
+    # See: https://developers.hubspot.com/docs/methods/oauth2/refresh-access-token
+    expire_in_seconds = 6 * 60 * 60
+    loop.run_until_complete(redis_client.set(key, token, expire=expire_in_seconds))
+
+# This client will share oauth2 credentials with other clients configured in the same way.
+hubspot3_client = Hubspot3(
+    access_token=access_token,
+    client_id=client_id,
+    client_secret=client_secret,
+    refresh_token=refresh_token,
+    oauth2_token_getter=oauth2_token_getter,
+    oauth2_token_setter=oauth2_token_setter,
+)
+```
+
 # Testing
 
 I'm currently working on rewriting many of the tests with