Update changelog

Author: Crozzers

CHANGES.md

@@ -14,6 +14,7 @@
 - [pull #665] Rewrite emphasis and strong processing to be more GFM compliant
 - [pull #672] Fix nested footnote references (#664)
 - [pull #681] Forbid square brackets in reference link IDs
+- [pull #687] Fix AssertionError hashing HTML blocks spread over multiple lines (#686)
 
 
 ## python-markdown2 2.5.4