This looks like a clearly written, compact, and easy-to-use neural network library. It could be useful for pedagogy, but the README should also mention (unless I missed it) for naive readers that it is pretty much completely inappropriate for production since it uses no vector instructions nor GPU computation. It will be
slow compared to any more sophisticated neural network library for anything but the most trivial applications.
For a similar equivalent in Swift see my project:
https://github.com/davecom/SwiftSimpleNeuralNetwork