Added note about Emacs version compatibility.

README

@@ -39,6 +39,14 @@ use the standard Emacs file local variable feature to embed the mode
 name into the first line of a file.  See the included example in
 examples/timedone.s for details.
 
+Also, be aware that this code was written with Gnu Emacs 24.3 so I may
+have used features not found in earlier versions.  If you see strange
+errors when you try to use it, check your Emacs version with
+
+  M-x emacs-version
+
+and be prepared to upgrade your Emacs if necessary.
+
 
 REFERENCES
 ==========