Add breaking change doc

Author: wilcobmsft

src/ResourceManager/Subscription/documentation/current-breaking-changes.md

@@ -38,4 +38,27 @@
     https://github.com/Azure/azure-powershell/blob/dev/documentation/breaking-changes/breaking-change-template.md
 -->
 
-## Current Breaking Changes
+## Current Breaking Changes
+The following cmdlets were affected this release:
+
+**Get-AzureRmSubscriptionDefinition**
+- Removed. Use Get-AzureRmSubscription instead.
+
+```powershell
+# Old
+# Get-AzureRmSubscriptionDefinition
+
+# New
+# Get-AzureRmSubscription
+```
+
+**New-AzureRmSubscriptionDefinition**
+- Replaced by New-AzureRmSubscription.
+
+```powershell
+# Old
+# New-AzureRmSubscriptionDefinition -Name mysubdef -OfferType ...
+
+# New
+# New-AzureRmSubscription -EnrollmentAccountObjectId ... -OfferType ...
+```