Adding a license

[FelixStach]

Hi Spandan,

This is probably the best minimal demo of skeletal animation with Assimp and OpenGL that I have come across so far. I really appreciate the work you've put into this project!

I wanted to ask if you could consider adding a license to the repository, such as the MIT License. Having a clear license would help others understand how they can use the project.

Best Regards,
Felix