add gosdk version in readme.go.md (#14635)

Author: chenjianfei2017

specification/cosmos-db/resource-manager/readme.go.md

@@ -13,6 +13,7 @@ go:
 
 ``` yaml $(go) && $(multiapi)
 batch:
+  - tag: package-2021-05
   - tag: package-2021-04
   - tag: package-2021-04-preview
   - tag: package-2021-03
@@ -28,6 +29,15 @@ batch:
   - tag: package-2015-04
 ```
 
+### Tag: package-2021-05 and go
+
+These settings apply only when `--tag=package-2021-05 --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-2021-05' && $(go)
+output-folder: $(go-sdk-folder)/services/cosmos-db/mgmt/2021-05-15/$(namespace)
+```
+
 ### Tag: package-2021-04 and go
 
 These settings apply only when `--tag=package-2021-04 --go` is specified on the command line.