Added version 2.6.6 to change log.

Fixes #475.

Author: waylan

docs/change_log.txt

@@ -7,6 +7,8 @@ next_url: release-2.6.html
 Python-Markdown Change Log
 =========================
 
+Mar 20, 2016: Released version 2.6.6 (a bug-fix release).
+
 Nov 24, 2015: Released version 2.6.5 (a bug-fix release).
 
 Nov 6, 2015: Released version 2.6.4 (a bug-fix release).