Add Format Preview to change log

Format Preview was the other huge thing in this last year, so lets
be sure to mention it. All of the other myriad changes can be
gleaned from 'git log --oneline R201601..HEAD'

SETUP/change_log.txt

@@ -129,9 +129,10 @@
     * various places: _() wrappers around strings to allow translation (lvl, cpeel)
     * various places: Miscellaneous improvements (jmdyck, lmpryor, cpeel, donovang, mike_cie, rfarinha)
 
-== Changes from R201601 to R?????? ==
+== Changes from R201601 to R201701 ==
 (Scripts supporting this upgrade are in SETUP/upgrade/09)
 
+    * MAJOR: New Format Preview functionality (rp31)
     * MAJOR: Removed dependency on magic quotes allowing PHP 5.4 and later support (cpeel)
 
 # vim: sw=4 ts=4 expandtab