add http-data-source to policy list

Author: dlepow

articles/api-management/api-management-policies.md

@@ -74,6 +74,7 @@ More information about policies:
 -  [Trigger output binding](invoke-dapr-binding-policy.md): Uses Dapr runtime to invoke an external system via output binding. To learn more about bindings in Dapr, see the description in this [README](https://github.com/dapr/docs/blob/master/README.md) file.
 
 ## GraphQL resolver policies
+- [HTTP data source for resolver](http-data-source-policy.md) - Configures the HTTP request and optionally the HTTP response to resolve data for an object type and field in a GraphQL schema.
 - [Publish event to GraphQL subscription](publish-event-policy.md) - Publishes an event to one or more subscriptions specified in a GraphQL API schema. Used in the `http-response` element of the `http-data-source` policy
 
 ## Transformation policies