Update readme.md (Azure#11564)

Author: arolshan

specification/monitor/resource-manager/readme.md

@@ -61,6 +61,7 @@ input-file:
 - Microsoft.Insights/stable/2016-03-01/alertRules_API.json
 - Microsoft.Insights/stable/2016-03-01/logProfiles_API.json
 - Microsoft.Insights/preview/2017-05-01-preview/diagnosticsSettings_API.json
+- Microsoft.Insights/preview/2017-05-01-preview/subscriptionDiagnosticsSettings_API.json
 - Microsoft.Insights/preview/2017-05-01-preview/diagnosticsSettingsCategories_API.json
 - Microsoft.Insights/stable/2019-06-01/actionGroups_API.json
 - Microsoft.Insights/stable/2017-04-01/activityLogAlerts_API.json