Docs: note about Unix.select descriptor limit
Docs: note about Unix.select descriptor limit

Resolves #222.

[skip ci]

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