IoTHub: update readme.java.md (#11244)

Author: xseeseesee

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

@@ -17,6 +17,7 @@ output-folder: $(azure-libraries-for-java-folder)/azure-mgmt-iothub
 ``` yaml $(java) && $(multiapi)
 batch:
   - tag: package-2020-03
+  - tag: package-preview-2019-07  
   - tag: package-preview-2019-03
   - tag: package-2018-12-preview
   - tag: package-2018-04
@@ -39,6 +40,19 @@ directive:
       to: PrivateLinkResource
 ```
 
+### Tag: package-preview-2019-07
+
+These settings apply only when `--tag=package-preview-2019-07 --java` is specified on the command line.
+Please also specify `--azure-libraries-for-java-folder=<path to the root directory of your azure-sdk-for-java clone>`.
+
+``` yaml $(tag) == 'package-preview-2019-07' && $(java) && $(multiapi)
+java:
+  namespace: com.microsoft.azure.management.iothub.v2019_07_01_preview
+  output-folder: $(azure-libraries-for-java-folder)/sdk/iothub/mgmt-v2019_07_01_preview
+regenerate-manager: true
+generate-interface: true
+```
+
 ### Tag: package-preview-2019-03
 
 These settings apply only when `--tag=package-preview-2019-03 --java` is specified on the command line.