Add major update note

Author: Alan Plum

CHANGELOG.md

@@ -16,6 +16,11 @@ This driver uses semantic versioning:
 
 ## [8.0.0] - 2022-10-25
 
+This is a major release and breaks backwards compatibility.
+
+See [the migration guide](./MIGRATING.md#v6-to-v7) for detailed instructions
+for upgrading your code to arangojs v7.
+
 ### Removed
 
 - Removed Node.js 10 and Node.js 12 support