Mention bindings-dsl in readme

[flip111]

I'm new to FFI. I run this tool, i get a hsc file. I think i can just put it in the hsc2hs tool. But i get an error of a missing bindings.dsl.h file. So from this i conclude that this package is suppose to work together with https://github.com/jwiegley/bindings-dsl

Maybe mention this in the readme. As the step after getting a hsc file is not so obvious on first use.