Add upgrade note for removal of panel types

docs/releases/3.0.md

@@ -139,6 +139,10 @@ wagtail updatemodulepaths --diff  # show the changes to be made, without updatin
 wagtail updatemodulepaths  # actually update the files
 ```
 
+### Removal of special-purpose field panel types
+
+Within panel definitions on models, `StreamFieldPanel`, `RichTextFieldPanel`, `ImageChooserPanel`, `DocumentChooserPanel` and `SnippetChooserPanel` should now be replaced with `FieldPanel`. Additionally, `PageChooserPanel` can be replaced with `FieldPanel` if it does not use the `page_type` or `can_choose_root` arguments.
+
 ### `BASE_URL` setting renamed to `WAGTAILADMIN_BASE_URL`
 
 References to `BASE_URL` in your settings should be updated to [`WAGTAILADMIN_BASE_URL`](wagtailadmin_base_url). This setting was not previously documented, but was part of the default project template when starting a project with the `wagtail start` command, and specifies the full base URL for the Wagtail admin, for use primarily in email notifications.