Update changelog.txt for release v3.6.11-slp.

Author: Anselm Kruis

Stackless/changelog.txt

@@ -4,9 +4,10 @@ Stackless-Python News
 This file documents user visible changes to the Stackless extension of C-Python.
 For other changes see Misc/NEWS.
 
-What's New in Stackless 3.6.9 and 3.6.10?
-=========================================
+What's New in Stackless 3.6.9, 3.6.10 and 3.6.11?
+=================================================
 
+*Release date: 2021-04-18*  (3.6.11)
 *Release date: 2021-04-18*  (3.6.10)
 *Release date: 2019-08-03*  (3.6.9)