update release notes with Hallo removal

- resolves #6228

CHANGELOG.txt

@@ -13,6 +13,7 @@ Changelog
  * Remove UI code for legacy browser support: polyfills, IE11 workarounds, Modernizr (Thibaud Colas)
  * Remove redirect auto-creation recipe from documentation as this feature is now supported in Wagtail core (Andy Babic)
  * Remove IE11 warnings (Gianluca De Cola)
+ * Remove the legacy Hallo rich text editor as it has moved to an external package (LB (Ben Johnston))
  * Fix: When using `simple_translations` ensure that the user is redirected to the page edit view when submitting for a single locale (Mitchel Cabuloy)
  * Fix: When previewing unsaved changes to `Form` pages, ensure that all added fields are correctly shown in the preview (Joshua Munn)
  * Fix: When Documents (e.g. PDFs) have been configured to be served inline via `WAGTAILDOCS_CONTENT_TYPES` & `WAGTAILDOCS_INLINE_CONTENT_TYPES` ensure that the filename is correctly set in the `Content-Disposition` header so that saving the files will use the correct filename (John-Scott Atlakson)

docs/releases/2.17.md

@@ -27,6 +27,7 @@ Here are other changes related to the redesign:
  * Replace `content_json` `TextField` with `content` `JSONField` in `PageRevision` (Sage Abdullah)
  * Remove `replace_text` management command (Sage Abdullah)
  * Replace `data_json` `TextField` with `data` `JSONField` in `BaseLogEntry` (Sage Abdullah)
+ * Remove the legacy Hallo rich text editor as it has moved to an external package (LB (Ben Johnston))
 
 
 ### Bug fixes
@@ -57,3 +58,10 @@ Here are other changes related to the redesign:
 * The default empty value for the field has been changed from `""` to `{}`.
 * This change also affects `BaseLogEntry` subclasses, i.e. `PageLogEntry` and `ModelLogEntry`.
 * If you have a large number of objects for these models, running the migrations might take a while.
+
+### 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 Hallo editor](https://github.com/wagtail/wagtail-hallo) legacy 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.