Add warning to top of release notes

Author: dstansby

docs/release-notes.rst

@@ -6,6 +6,12 @@ Release notes
 3.0.8 (2025-05-19)
 ------------------
 
+.. warning::
+
+    In versions 3.0.0 to 3.0.7 opening arrays with ``mode='a'`` (the default for many builtin functions)
+    would cause any existing paths in the store to be deleted. This is fixed in 3.0.8, and
+    we recommend all users upgrade to avoid this bug that could cause unintentional data loss.
+
 Features
 ~~~~~~~~