mysql readme update for typescript (Azure#15963)

Author: DaeunYim

specification/mysql/resource-manager/readme.typescript.md

@@ -11,3 +11,24 @@ typescript:
   clear-output-folder: true
   generate-metadata: true
 ```
+
+### Tag: package-flexibleserver-2021-05-01 and TypeScript
+
+These settings apply only when `--tag=package-flexibleserver-2021-05-01 --typescript` is specified on the command line.
+Please also specify `--typescript-sdks-folder=<path to the root directory of your azure-sdk-for-typescript clone>`.
+
+``` yaml $(tag) == 'package-flexibleserver-2021-05-01' && $(typescript)
+title: MySQLManagementFlexibleServerClient
+package-name: "@azure/arm-mysql-flexible"
+output-folder: "$(typescript-sdks-folder)/sdk/mysql/arm-mysql-flexible/"
+```
+
+### Tag: package-2020-01-01 and TypeScript
+
+These settings apply only when `--tag=package-2020-01-01 --typescript` is specified on the command line.
+Please also specify `--typescript-sdks-folder=<path to the root directory of your azure-sdk-for-typescript clone>`.
+
+``` yaml $(tag) == 'package-2020-01-01' && $(typescript)
+package-name: "@azure/arm-mysql"
+output-folder: "$(typescript-sdks-folder)/sdk/mysql/arm-mysql/"
+```