add sway LICENSE file

part of the code in dwl is taken from sway, so credit it.
dwm and sway are both licensed under the MIT license

LICENSE

@@ -2,7 +2,7 @@ dwl - dwm for Wayland
 
 Copyright © 2020 dwl team
 
-See also the files LICENSE.tinywl and LICENSE.dwm.
+See also the files LICENSE.tinywl, LICENSE.dwm and LICENSE.sway.
 
 This program is free software: you can redistribute it and/or modify
 it under the terms of the GNU General Public License as published by

LICENSE.sway

@@ -0,0 +1,19 @@
+Copyright (c) 2016-2017 Drew DeVault
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of
+this software and associated documentation files (the "Software"), to deal in
+the Software without restriction, including without limitation the rights to
+use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies
+of the Software, and to permit persons to whom the Software is furnished to do
+so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.