Added license to README.md

Author: josejuansanchez

README.md

@@ -55,6 +55,11 @@ board.ledmatrix.print("H o l a  m u n d o!");
 
 This library has been developed by [Antonio Morales][2] and [José Juan Sánchez][3].
 
+## License
+
+This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.
+
+This library 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 Lesser General Public License for more details.
 
 [1]: http://github.com/AntonioMR/Arduino-Nano-Play-Board
 [2]: http://twitter.com/antonio1010mr