Update readme.python.md (#15818)

Author: BigCat20196

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

@@ -17,6 +17,7 @@ Generate all API versions currently shipped for this package
 ```yaml $(multiapi)
 clear-output-folder: true
 batch:
+  - tag: package-2021-03
   - tag: package-2021-02
   - tag: package-2020-11
   - tag: package-2020-08
@@ -59,6 +60,16 @@ output-folder: $(python-sdks-folder)/network/azure-mgmt-network/azure/mgmt/netwo
 clear-output-folder: false
 perform-load: false
 ```
+### Tag: package-2021-03 and python
+
+These settings apply only when `--tag=package-2021-03 --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-2021-03'
+namespace: azure.mgmt.network.v2021_03_01
+output-folder: $(python-sdks-folder)/network/azure-mgmt-network/azure/mgmt/network/v2021_03_01
+```
+
 ### Tag: package-2021-02 and python
 
 These settings apply only when `--tag=package-2021-02 --python` is specified on the command line.