updated readme with SKU defination

Author: gargankit-microsoft

specification/workloads/resource-manager/readme.md

@@ -40,6 +40,7 @@ input-file:
   - Microsoft.Workloads/preview/2021-12-01-preview/SAPVirtualInstance.json
   - Microsoft.Workloads/preview/2021-12-01-preview/operations.json
   - Microsoft.Workloads/preview/2021-12-01-preview/monitors.json
+  - Microsoft.Workloads/preview/2021-12-01-preview/skus.json
 ```
 
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