Update changelog for Vaadin 8.15.1 (#12536)

CHANGELOG-VAADIN8.md

@@ -1,5 +1,11 @@
 # Vaadin 8 extended maintenance version changelog
 
+## Vaadin 8.15.1
+
+* Fixed an issue where Grid was moving focus away from external input controls when the datasource contents were updated
+* Fixed an issue where manual field binding configurations might get overwritten by automatic binding logic
+* Field level verification in Binder is no longer run twice
+
 ## Vaadin 8.15.0
 
 * Change license from Apache 2.0 to CVDLv4