Issue backdrop#4485: Move LICENSE.txt in the project root.

By @klonos and @quicksketch.

Author: klonos

core/LICENSE.txt LICENSE.txtcore/LICENSE.txt renamed to LICENSE.txt

@@ -80,8 +80,8 @@ Please see the the [Backdrop API Documentation](https://api.backdropcms.org).
 License
 -------
 Backdrop is [GPL v2](http://www.gnu.org/licenses/gpl-2.0.html) (or higher)
-software. See the LICENSE.txt file in the core directory for complete text.
-Distributions of this software may relicense it as any later version of the GPL.
+software. See the LICENSE.txt file for complete text. Distributions of this
+software may relicense it as any later version of the GPL.
 
 All Backdrop code is Copyright 2001 - 2016 by the original authors.