add license header

smux

@@ -1,10 +1,21 @@
 #!/bin/sh
-
-###################################################################################################
-# ssh+tmux = smux
-# 
-#   a light, POSIX wrapper around ssh to create and attach to tmux sessions on remote computers
-###################################################################################################
+# SPDX-License-Identifier: GPL-3.0-only
+#
+# This file is part of the smux project: https://github.com/tomeichlersmith/smux
+#
+# Copyright (C) 2021 smux contributors
+#
+# smux is free software; you can redistribute it and/or modify it
+# under the terms of the GNU General Public License version 3
+# as published by the Free Software Foundation.
+#
+# smux 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 smux; if not, see <http://www.gnu.org/licenses/>.
 
 # POSIX
 set -o errexit