Update API version history doc

Info copied from magnum/api/controllers/versions.py

Change-Id: I8e266e49b6d3cfe5a0920bf2346fd743e0365281

magnum/api/rest_api_version_history.rst

@@ -77,6 +77,26 @@ user documentation.
   A non-admin user can get self quota information.
 
 
+1.7
+---
+
+  Add resize API
+
+
+1.8
+---
+
+  Add upgrade API
+
+
+1.9
+---
+
+  Add nodegroup API
+
+  Allow create/update/delete/list of default-worker and additional nodegroups.
+
+
 1.10
 ---