added short license information to script

letsencrypt.sh

@@ -2,6 +2,8 @@
 
 # letsencrypt.sh by lukas2511
 # Source: https://github.com/lukas2511/letsencrypt.sh
+#
+# This script is licensed under The MIT License (see LICENSE for more information).
 
 set -e
 set -u