Add GPLv3 license file

[HaydenNess]

Hi,

If you've intended this project to be released under GPLv3, as would be inherited from YOLOv7 and DeepSORT, please consider adding a license file to the repository.

I am working on a commerical application (non-monetary, not distributed) and would like to make use of this project but want to be legally clear.

Thanks.

[moooises]

Yes, please. It would help a lot.

[deshwalmahesh]

Can you send me that here or maybe open up a request. I'll merge