add License

as suggested in http://choosealicense.com/licenses/bsd-3-clause/

based on license information given in https://github.com/2amigos/yiiwheels/blob/master/composer.json and https://github.com/2amigos/yiiwheels/issues/22

@tonydspaniard 
Have seen that you changes copyright to 2Amigos in yiiwheels-doc - of course you decide if you want to add your name or company to the copyright notice.
Afaik most use  the personal name in the copyright notice and the name of the company instead of {organization} in the text (I'm no lawyer...).