Please add a LICENSE + A Rust Implementation

[suyash]

Please see https://github.com/suyash/grokking-deep-learning-rs

I have implemented all the exercises in Rust. Rust is a new programming language with a focus on safety, speed and concurrency, while having a minimal runtime and zero garbage collection.

In the Rust community, it is common to dual license code with Apache 2 and MIT. I have done the same. Hopefully that's okay.

[dimchansky]

Thanks for the implementation in the Rast language. You also noticed incorrectly calculated weights in Chapter 5: https://github.com/suyash/grokking-deep-learning-rs/blob/master/examples/chapter5.rs#L215

I've registered issue for this: #23