add history

Author: reneeli@microsoft.com

src/aks-preview/HISTORY.rst

@@ -27,6 +27,7 @@ Pending
 14.0.0b6
 +++++++
 * Add option `--acns-advanced-networkpolicies <None|FQDN|L7>` to `az aks create/update`
+* Add option `--migrate-vmas-to-vms` to `az aks update`
 
 14.0.0b5
 ++++++++