Add a license to the repository

[cbjuan]

I recently came across your project hamiltonian_engine and I noticed that the repository github.com/nus-comparch/hamiltonian_engine currently does not have a license specified.

Why Adding a License is Important?

As GitHub documents

Public repositories on GitHub are often used to share open source software. For your repository to truly be open source, you'll need to license it so that others are free to use, change, and distribute the software.

Would be possible to add a valid license to this repository? Probably something really OpenSource such as the Apache 2.0 license would fit really well.

Thanks!