update release notes with Hallo removal

- resolves #6228

docs/releases/2.17.md

@@ -29,3 +29,9 @@ Here are other changes related to the redesign:
 
 ## Upgrade considerations
 
+### Hallo legacy rich text editor has moved to an external package
+
+* Hallo was deprecated in [Wagtail v2.0 (February 2018)](https://docs.wagtail.org/en/stable/releases/2.0.html#new-rich-text-editor) and has had only a minimal level of support 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-hallo) package.
+* We encourage all users of the Hallo editor to take steps to migrate to the new Draftail editor as this external package is unlikely to have ongoing maintenance.
+* `window.registerHalloPlugin` will no longer be created on the page editor load, unless the legacy package is installed.