Add migration guide for Stack AzureRm 1.2.11

[viananth]

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[markcowl]

I think you will want to add in the changes from the migration guides for 2.X and 3.X powershell for the modules you are using (Profile esp.): https://github.com/Azure/azure-powershell/blob/preview/documentation/release-notes/migration-guide.2.0.0.md, https://github.com/Azure/azure-powershell/blob/preview/documentation/release-notes/migration-guide.3.0.0.md, https://github.com/Azure/azure-powershell/blob/preview/documentation/release-notes/migration-guide.4.0.0.md

[bganapa]

We also have changes in the Get-AzureRmSubscription and Select-AzureRmSubscription cmdlets differring in outputs from 1.2.10

We cannot pipe azurestack subscription model to Select-AzureRmSubscription, we need to call these out

[bganapa]

@markcowl vishnu added the Profile Changes from 3.0 and 4.0

[bganapa]

Thanks again!