Add new api-version of policy for go SDK

Author: ArcturusZhang

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

@@ -25,6 +25,7 @@ batch:
   - tag: package-features-2015-12
   - tag: package-locks-2016-09
   - tag: package-locks-2015-01
+  - tag: package-policy-2020-03
   - tag: package-policy-2019-09
   - tag: package-policy-2019-06
   - tag: package-policy-2019-01
@@ -108,6 +109,16 @@ namespace: policy
 output-folder: $(go-sdk-folder)/services/resources/mgmt/2019-09-01/policy
 ```
 
+### Tag: package-policy-2020-03 and go
+
+These settings apply only when `--tag=package-policy-2020-03 --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-policy-2020-03' && $(go)
+namespace: policy
+output-folder: $(go-sdk-folder)/services/preview/resources/mgmt/2020-03-01-preview/policy
+```
+
 ### Tag: package-policy-2019-06 and go
 
 These settings apply only when `--tag=package-policy-2019-06 --go` is specified on the command line.