Mention the hash table change.

NEWS

@@ -242,6 +242,13 @@ Changes since 2.6.6:
 
     - Added the --copy-dirlinks option, a more limited version of --copy-links.
 
+    - The sender's hash table for checksum information was improved to make it
+      more efficient when updating really large files (starting from around
+      2.5GB, but most visibly for much larger files).  This prevents the hash
+      table from getting overloaded, which saves CPU time (and real time) at
+      the expense of a little extra memory (and it actually saves memory for
+      normal sized files).
+
     - Various documentation improvements, including: a better synopsis, some
       improved examples, a better discussion of the presence and absence of
       --perms (including how it interacts with the new --executability and