Add new api-version for databoxedge

Author: ArcturusZhang

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

@@ -13,11 +13,21 @@ go:
 
 ```yaml $(go) && $(multiapi)
 batch:
+  - tag: package-2020-05-preview
   - tag: package-2019-08
   - tag: package-2019-07
   - tag: package-2019-03  
 ```
 
+### Tag: package-2020-05-preview and go
+
+These settings apply only when `--tag=package-2020-05-preview --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-2020-05-preview' && $(go)
+output-folder: $(go-sdk-folder)/services/preview/$(namespace)/mgmt/2020-05-01-preview/$(namespace)
+```
+
 ### Tag: package-2019-08 and go
 
 These settings apply only when `--tag=package-2019-08 --go` is specified on the command line.