include usage comment in snomasks.pl

scripts/snomasks.pl

@@ -1,5 +1,9 @@
 # Released into the Public Domain
 
+# Note: After loading the script and adding snomasks into one of your bars, you
+# must request /umode once on each server you have server notices masks set on.
+# After that, the script will automatically update the bar item.
+
 use strict;
 use warnings;