changes.src: document Win64 build

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
diff --git a/doc/changes.src b/doc/changes.src
index 94ddb9c..c6738b9 100644
--- a/doc/changes.src
+++ b/doc/changes.src
@@ -40,6 +40,8 @@
 \b Line numbers in list files now correspond to the lines in the source
    files, instead of simply being sequential.
 
+\b There is now an official 64-bit (x64 a.k.a. x86-64) build for Windows.
+
 
 \S{cl-2.11.09} Version 2.11.09