message: add tag into readme.go.md
diff: @@ -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.
  