docs: update embedded readme with user params context (apache#25800)

Author: jbat

superset-embedded-sdk/README.md

@@ -76,6 +76,10 @@ Guest tokens can have Row Level Security rules which filter data for the user ca
 
 The agent making the `POST` request must be authenticated with the `can_grant_guest_token` permission.
 
+Within your app, using the Guest Token will then allow authentication to your Superset instance via creating an Anonymous user object.  This guest anonymous user will default to the public role as per this setting `GUEST_ROLE_NAME = "Public"`.
++
++The user parameters in the example below are optional and are provided as a means of passing user attributes that may be accessed in jinja templates inside your charts.
+
 Example `POST /security/guest_token` payload:
 
 ```json