docs: add service-app token quick start

Author: Adam Weeks

README.md

@@ -17,18 +17,28 @@ Until publication, install from a local checkout:
 python3 -m pip install -e .
 ```
 
-## Authenticate
+## Quick start with a service-app token
 
-For an already-issued service-app token, use a static provider:
+1. Create and authorize a [Webex Service App](https://developer.webex.com/create/docs/service-apps).
+2. Select `spark-admin:datasource_read` to list data sources and
+   `spark-admin:datasource_write` to register, update, or extend them. See the
+   [Data Sources API reference](https://developer.webex.com/webex-contact-center/docs/api/v1/data-sources)
+   for the required permissions.
+3. Retrieve the authorized service app's access token and pass it directly to
+   the SDK from your application's secret source. Do not commit it or embed it
+   in source code.
 
 ```python
 from webex_byods import StaticAccessTokenProvider, WebexDataSourceClient
 
 client = WebexDataSourceClient(
-    token_provider=StaticAccessTokenProvider("service-app-access-token")
+    token_provider=StaticAccessTokenProvider(service_app_access_token)
 )
+data_sources = client.list_all_data_sources()
 ```
 
+## Refresh service-app tokens
+
 For automatic service-app token acquisition, the host application supplies
 credentials and a personal-token provider. It can obtain those values from its
 own configuration, AWS Secrets Manager, or another secret system.