COPYING: add COPYING file to specify project licenses

Since multiple licenses are provided now in the LICENSES folder,
add a COPYING file to specify the project license.

Signed-off-by: Adrian Schmutzler <[email protected]>
Acked-by: Paul Spooren <[email protected]>

COPYING

@@ -0,0 +1,12 @@
+OpenWrt is provided under:
+
+	SPDX-License-Identifier: GPL-2.0-only
+
+Being under the terms of the GNU General Public License version 2 only,
+according with:
+
+	LICENSES/GPL-2.0
+
+In addition, other licenses may also apply.
+
+All contributions to OpenWrt are subject to this COPYING file.