cmd/rlpdump: add license header and synopsis

cmd/rlpdump/main.go

+/*
+	This file is part of go-ethereum
+
+	go-ethereum is free software: you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation, either version 3 of the License, or
+	(at your option) any later version.
+
+	go-ethereum is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License
+	along with go-ethereum.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+// rlpdump is a pretty-printer for RLP data.
 package main
 
 import (