docker-compose.yml: added comment suggesting ways to share x-server

docker-compose.yml

@@ -1,7 +1,9 @@
 version: '2'
 
-# TODO consider passing $XAUTHORITY
-# https://stackoverflow.com/a/25280523/5894777
+# options to share host's x-server:
+# - Xephyr
+# - pass $XAUTHORITY (insecure, https://stackoverflow.com/a/25280523/5894777)
+# - xhost + (horribly insecure)
 
 services:
   browser: