Added the text of the GNU GPL 3

I saw you copy pasted a lot of @OpenRA code https://github.com/Phrohdoh/OSShpBuilder/commit/0abab6ba7cd28227cc5b9c7e24ef93078bd9ea7c, but forgot to add the full text of the license (this is required: https://www.gnu.org/licenses/gpl-faq.html#WhyMustIInclude) and give proper attribution. Please note that OpenRA is not LGPL which makes your whole program GPL v3 licensed.