Update readme.md (#14933)

Add R4007 suppression for metricNamespaces since changing it's return type would be a breaking change

Author: ToddKingMSFT

specification/monitor/resource-manager/readme.md

@@ -722,6 +722,9 @@ directive:
   - suppress: DefaultErrorResponseSchema
     from: activityLogAlerts_API.json
     reason: 'Updating the error response to the new format would be a breaking change.'
+  - suppress: DefaultErrorResponseSchema
+    from: metricNamespaces_API.json
+    reason: 'Updating the error response to the new format would be a breaking change.'
   - suppress: DefaultErrorResponseSchema
     from: metrics_API.json
     reason: 'Updating the error response to the new format would be a breaking change.'