set default tag for resource-manager to resources (Azure#18475)

Author: cataggar

specification/resources/resource-manager/readme.md

@@ -26,6 +26,7 @@ These are the global settings for the Resource API.
 
 ``` yaml
 openapi-type: arm
+tag: package-resources-2021-04
 ```
 
 ``` yaml $(package-privatelinks)