add history

Author: Hao Yuan

src/aks-preview/HISTORY.rst

@@ -11,11 +11,14 @@ To release a new version, please select a new version number (usually plus 1 to
 
 Pending
 +++++++
+* Add support for `ManagedSystem` Agent Pool Mode.
+
 18.0.0b18
 +++++++
 * Add validation error when neither --location or --cluster and --resource-group-name are specified for az extension type list or az extension type version list
 
 
+
 18.0.0b17
 +++++++
 * Remove `__import__('pkg_resources').declare_namespace(__name__)` from `vendored_sdks/__init__.py`` to fix the namespace package issue.