Document macho64 output format

At least mention the 64-bit macho format in the documentation.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/doc/changes.src b/doc/changes.src
index 1920b5b..4a15ae7 100644
--- a/doc/changes.src
+++ b/doc/changes.src
@@ -35,6 +35,8 @@
 \b Don't delete the list file on errors.  Also, include error and
   warning information in the list file.
 
+\b Support for 64-bit Mach-O output, see \k{machofmt}.
+
 
 \S{cl-2.06} Version 2.06