MINOR: add upgrade note about breadcrumbs

Author: wilr

docs/en/changelogs/3.0.0.md

@@ -136,6 +136,12 @@ unless these are explicitly noted.
 
 Alternatively, you can enforce database usage by setting `SapphireTest->usesDatabase` to `TRUE` in your test class.
 
+### Breadcrumbs ###
+
+Breadcrumbs have been altered to be their own template. In the process of this, the common static
+SiteTree::$breadcrumbs_delimiter has been removed. To customise breadcrumbs now, create a template
+BreadcrumbsTemplate.ss from cms/template to your theme or application.
+
 ### Deprecated Classes ###
 
  * `FileIframeField`: Use `UploadField`