update java readme digitaltwins (Azure#9878)

Author: ChenTanyi

specification/digitaltwins/resource-manager/readme.java.md

@@ -0,0 +1,33 @@
+## Java
+
+These settings apply only when `--java` is specified on the command line.
+Please also specify `--azure-libraries-for-java-folder=<path to the root directory of your azure-libraries-for-java clone>`.
+
+``` yaml $(java)
+azure-arm: true
+fluent: true
+namespace: com.microsoft.azure.management.digitaltwins
+license-header: MICROSOFT_MIT_NO_CODEGEN
+payload-flattening-threshold: 1
+output-folder: $(azure-libraries-for-java-folder)/azure-mgmt-digitaltwins
+```
+
+### Java multi-api
+
+``` yaml $(java) && $(multiapi)
+batch:
+  - tag: package-2020-03-01-preview
+```
+
+### Tag: package-2020-03-01-preview and java
+
+These settings apply only when `--tag=package-2020-03-01-preview --java` is specified on the command line.
+Please also specify `--azure-libraries-for-java=<path to the root directory of your azure-sdk-for-java clone>`.
+
+``` yaml $(tag) == 'package-2020-03-01-preview' && $(java) && $(multiapi)
+java:
+  namespace: com.microsoft.azure.management.digitaltwins.v2020_03_01_preview
+  output-folder: $(azure-libraries-for-java-folder)/sdk/digitaltwins/mgmt-v2020_03_01_preview
+regenerate-manager: true
+generate-interface: true
+```

specification/digitaltwins/resource-manager/readme.md

@@ -78,6 +78,10 @@ See configuration in [readme.typescript.md](./readme.typescript.md)
 
 See configuration in [readme.csharp.md](./readme.csharp.md)
 
+## Java
+
+See configuration in [readme.java.md](./readme.java.md)
+
 ## Multi-API/Profile support for AutoRest v3 generators
 
 AutoRest V3 generators require the use of `--tag=all-api-versions` to select api files.