meta: make copyright headers more maintainable

- list all files as copyright "The River Developers"
- add an AUTHORS file to acknowledge contributors

example/status.zig

@@ -1,6 +1,6 @@
 // This file is part of river, a dynamic tiling wayland compositor.
 //
-// Copyright 2020 Isaac Freund
+// Copyright 2020 The River Developers
 //
 // 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