adds note about breaking changes to migration guide

Author: stockiNail

docs/guide/migrationV2.md

@@ -1 +1,10 @@
 # 2.x Migration guide
+
+`chartjs-plugin-annotation` plugin version 2 introduces a number of breaking changes. In order to improve performance, offer new features, and improve maintainability, it was necessary to break backwards compatibility, but we aimed to do so only when worth the benefit.
+
+## Options
+
+A number of changes were made to the configuration options passed to the plugin configuration. Those changes are documented below.
+
+ * `xScaleID` option default has been changed, now set to `undefined`. If the option is missing, the plugin will try to use the first scale of the chart, configured as `'x'` axis. If more than one scale has been defined in the chart as `'x'` axis, the option is mandatory to select the right scale.
+ * `yScaleID` option default has been changed, now set to `undefined`. If the option is missing, the plugin will try to use the first scale of the chart, configured as `'y'` axis. If more than one scale has been defined in the chart as `'y'` axis, the option is mandatory to select the right scale.