[Msi] update readme.python to Multiapi (Azure#18408)

* update readme.python to multiapi

* Update readme.python.md

* Update readme.python.md

specification/msi/resource-manager/readme.python.md

@@ -8,12 +8,44 @@ Use `--python-mode=update` if you already have a setup.py and just want to updat
 azure-arm: true
 license-header: MICROSOFT_MIT_NO_VERSION
 package-name: azure-mgmt-msi
-namespace: azure.mgmt.msi
 package-version: 1.0.0b1
+no-namespace-folders: true
+```
+
+### Python multi-api
+
+Generate all API versions currently shipped for this package
+
+
+```yaml $(multiapi) && $(python)
 clear-output-folder: true
+batch:
+  - tag: package-preview-2021-09
+  - tag: package-2018-11-30
+  - multiapiscript: true
 ```
 
-``` yaml $(python)
-no-namespace-folders: true
-output-folder: $(python-sdks-folder)/resources/azure-mgmt-msi/azure/mgmt/msi
+``` yaml $(multiapiscript)
+output-folder: $(python-sdks-folder)/resources/azure-mgmt-msi/azure/mgmt/msi/
+perform-load: false
+```
+
+### Tag: package-2019-09-preview and python
+
+These settings apply only when `--tag=package-preview-2021-09 --python` is specified on the command line.
+Please also specify `--python-sdks-folder=<path to the root directory of your azure-sdk-for-python clone>`.
+
+``` yaml $(tag) == 'package-preview-2021-09'
+namespace: azure.mgmt.msi.v2019_09_01_preview
+output-folder: $(python-sdks-folder)/resources/azure-mgmt-msi/azure/mgmt/msi/v2019_09_01_preview
+```
+
+### Tag: package-2018-11-30 and python
+
+These settings apply only when `--tag=package-2018-11-30 --python` is specified on the command line.
+Please also specify `--python-sdks-folder=<path to the root directory of your azure-sdk-for-python clone>`.
+
+``` yaml $(tag) == 'package-2018-11-30'
+namespace: azure.mgmt.msi.v2018_11_30
+output-folder: $(python-sdks-folder)/resources/azure-mgmt-msi/azure/mgmt/msi/v2018_11_30
 ```