Add issue number to changelog

Related to #1863.

(cherry picked from commit 33cd8b865d9289cfbcf3d95e6e68e4050b94fcd3)
diff --git a/README.md b/README.md
index 1291216..76907a1 100644
--- a/README.md
+++ b/README.md
@@ -119,6 +119,7 @@
 ## Changelog
 
  - [X11] Bugfix: Some window attributes were not applied on leaving fullscreen
+   (#1863)
 
 
 ## Contact