Readme / example improvement

[patrick-nicodemus]

I am not experienced with compiling OCaml with C dependencies so I had some pretty basic misunderstandings while trying to get this set up. This is basically just a problem with me misunderstanding how dune works in this scenario but other newbies might have the same issue.
The futhark-bindgen help --backend xyz commands are very helpful, but do you think you could futher clarify (either in the OCaml example section or in the help command itself or both) whether the given flags are supposed to be fed to library_flags or c_library_flags?
I initially thought that all the flags given by the --backend xyz were supposed to be given to library_flags and had to consult the dune documentation for a while to resolve my misunderstanding.