Update draft release notes to 1.7.6.6

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes/1.7.6.6.txt

@@ -8,4 +8,9 @@ Fixes since v1.7.6.5
    directory when two paths in question are in adjacent directories and
    the name of the one directory is a prefix of the other.
 
+ * When producing a "thin pack" (primarily used in bundles and smart
+   HTTP transfers) out of a fully packed repository, we unnecessarily
+   avoided sending recent objects as a delta against objects we know
+   the other side has.
+
 Also contains minor fixes and documentation updates.