update changelog (#687)

Author: ocefpaf

CHANGES.txt

@@ -9,7 +9,8 @@
 - Added support for subdomains options in TileLayer (damselem #623)
 - Updated to leaflet 1.1.0 (ocefpaf #639)
 - Added `FastMarkerCluster` (James Gardiner #585 (proposed by @ruoyu0088))
-- Changed default css image-rendering style for image overlays to pixelated
+- Use the GIS standard "pixelated" css image-rendering in
+  image overlays by default (dirkvdb #684 and ocefpaf #686)
 
 API changes