docs: Update README.md with new permissions (#518)

README.md

@@ -180,11 +180,11 @@ an AlloyDB instance. The proxy supports the following options:
 3. [Application Default Credentials (ADC)][adc] if neither of the above have
    been set.
 
-Note: Any account connecting to AlloyDB instance will need one of the following
-IAM roles:
+Any principal connecting to an AlloyDB instance will need the following
+IAM roles or equivalent permissions:
 
-- Cloud AlloyDB Client (`roles/alloydb.client`) (preferred)
-- Cloud AlloyDB Admin (`roles/alloydb.admin`)
+- Cloud AlloyDB Client (`roles/alloydb.client`)
+- Service Usage Consumer (`roles/serviceusage.serviceUsageConsumer`)
 
 See [Roles and Permissions in AlloyDB][roles-and-permissions] for details.