Lwt_io: warn about type safety of read_value
Lwt_io: warn about type safety of read_value

This commit adds some temporary documentation to Lwt_io.read_value and
Lwt_io.write_value, to warn users about the potential dangers of using
the underlying Marshal module. Better documentation will be written at
some future date, perhaps as part of writing a manual for the Unix
binding.

Resolves #345.

ocsigen/lwt,c-cube/lwt,rneswold/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,ocsigen/lwt,c-cube/lwt,rneswold/lwt