Add usage

main.go

@@ -26,12 +26,20 @@ import (
 var Version string
 var apiToken string
 
+var usage string = `Telegram List bot
+Environment variables:
+TGLB_API_TOKEN (required): See https://core.telegram.org/bots#how-do-i-create-a-bot
+TGLB_PORT (default 28923): Set numerical port or unix//run/path.sock for unix socket
+TGLB_DATA_PATH (default .): Directory path where list data is stored
+`
+
 func main() {
 
 	apiToken = os.Getenv("TGLB_API_TOKEN")
 
 	if apiToken == "" {
-		log.Panicln("TG_API_TOKEN is empty")
+		log.Print(usage)
+		log.Panicln("TGLB_API_TOKEN is empty")
 	}
 
 	port, unixSocketPath := func() (int, string) {