Update u_e-sogo.md

docs/u_e-sogo.md

@@ -33,6 +33,7 @@ and replace with:
 
 ## Change favicon
 mailcow builds after 30 January 2021 can change SOGo's favicon by replacing `data/conf/sogo/custom-favicon.ico`.
+To note: you can use `.png` favicons, renaming them `custom-favicon.ico` will works, but please use standard `.ico` dimensions, e.g: 16x16, 32x32, 64x64, 128x128 and 256x256.
 After you replaced said file you need to restart SOGo and Memcached containers by executing `docker-compose restart memcached-mailcow sogo-mailcow`.
 
 ## Change logo