ADD GPLv3

[MaKaNu]

I recognized the missing LICENSE file and the open Issue #6. I was at the same Situation as you: Having a Repo without a License. The first Time I needed to add a LICENSE, I struggled with choosing the right one. Over the Time I have learned about when and why to choose which license text. Github actually explains nowadays pretty well what each license stands for, but I like to help you further.

This is the first PR of three in relation to Licenses:

GPLv3:

• I use this License whenever I truly want to support OpenSource and OpenSource only (most of my cases)
• With the copy-left restriction I prohibit only binary shares of my code
• This might exclude commercial use and corporation usage

Final Words: I am not a lawyer, you might want to investigate which License fits best for your purposes and decide then. But if your code is meant to be OpenSource a license text is mandatory.