update release notes with Hallo removal

docs/releases/2.17.md

@@ -27,3 +27,9 @@ Here are other changes related to the redesign:
 
 ## Upgrade considerations
 
+### Hallo legacy rich text editor has moved to an external app
+
+* Hallo was deprecated in in Wagtail version 2.0 (February 2018) and has since not received any bug fixes but has had a minimum level of support in Wagtail versions since then.
+* If you still require Hallo for your Wagtail installation, you will need to install the [Wagtail legacy Hallo editor](https://github.com/wagtail/wagtail-legacy-hallo-editor) app.
+* There will be no ongoing support for this package beyond this release.
+* `window.registerHallowPlugin` will no longer be created on the page editor load unless the legacy package is installed.