add changelog entry for construct_whitelister_element_rules hook

CHANGELOG.txt

@@ -14,6 +14,7 @@ Changelog
  * Aesthetic improvements to preview experience
  * 'image' tag now accepts extra keyword arguments to be output as attributes on the img tag
  * Added an 'attrs' property to image rendition objects to output src, width, height and alt attributes all in one go
+ * Added 'construct_whitelister_element_rules' hook for customising the HTML whitelist used when saving rich text fields
  * Fix: Animated GIFs are now coalesced before resizing
  * Fix: Wand backend clones images before modifying them
  * Fix: Admin breadcrumb now positioned correctly on mobile