Add short comparison with hoppy to readme

[walseb]

Hi!
I'm currently researching what FFI library to use and I can't find much info on the differences between the various FFI generator approaches. Specifically I'm having a hard time choosing between this library and hoppy. The main difference I have noticed is that hoppy supports generating foreign pointers while this doesn't. Hoppy might also have less support for C++ templates? Anyways I think it would be useful and I would be grateful if a short comparison between this library and hoppy was added to the readme. Thanks for making this!