plugin/bind: Point to MacOS bug (#4995)

Document this is a bug.

Closes: 4993
Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/bind/README.md

@@ -101,3 +101,6 @@ bad.example.com {
     forward . 5.6.7.8
 }
 ```
+
+Also on MacOS there is an (open) bug where this doesn't work properly. See
+<https://github.com/miekg/dns/issues/724> for details, but no solution.