add tag into readme.go.md (Azure#15022)

Author: chenjianfei2017

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

@@ -13,6 +13,7 @@ go:
 
 ``` yaml $(go) && $(multiapi)
 batch:
+  - tag: package-preview-2021-01
   - tag: package-2020-12
   - tag: package-2019-12
   - tag: package-preview-2019-12
@@ -24,6 +25,15 @@ batch:
   - tag: package-2016-07
 ```
 
+### Tag: package-preview-2021-01 and go
+
+These settings apply only when `--tag=package-preview-2021-01 --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-preview-2021-01' && $(go)
+output-folder: $(go-sdk-folder)/services/preview/$(namespace)/mgmt/2021-01-01-preview/$(namespace)
+```
+
 ### Tag: package-2020-12 and go
 
 These settings apply only when `--tag=package-2020-12 --go` is specified on the command line.