add tags for go sdk release v55.0.0 (Azure#14567)

Author: ArcturusZhang

specification/cognitiveservices/resource-manager/readme.go.md

@@ -13,10 +13,20 @@ go:
 
 ``` yaml $(go) && $(multiapi)
 batch:
+  - tag: package-2021-04
   - tag: package-2017-04
   - tag: package-2016-02-preview
 ```
 
+### Tag: package-2021-04 and go
+
+These settings apply only when `--tag=package-2021-04 --go` is specified on the command line.
+Please also specify `--go-sdk-folder=<path to the root directory of your azure-sdk-for-go clone>`.
+
+``` yaml $(tag) == 'package-2021-04' && $(go)
+output-folder: $(go-sdk-folder)/services/$(namespace)/mgmt/2021-04-30/$(namespace)
+```
+
 ### Tag: package-2017-04 and go
 
 These settings apply only when `--tag=package-2017-04 --go` is specified on the command line.