update RELEASE_NOTES for node method_missing removal

Signed-off-by: Lamont Granquist <lamont@scriptkiddie.org>

RELEASE_NOTES.md

@@ -53,3 +53,9 @@ The remediation is removing the self-dependency `depends` line in the metadata.
 
 Retained only for the service resource (where it makes some sense) and for the mount resource.
 
+### Removed deprecated `method_missing` access from the Chef::Node object
+
+Previously, the syntax `node.foo.bar` could be used to mean `node["foo"]["bar"]`, but this API had sharp edges where methods collided
+with the core ruby Object class (e.g. `node.class`) and where it collided with our own ability to extend the `Chef::Node` API.  This
+method access has been deprecated for some time, and has been removed in Chef-13.
+