Docs: Using Lwt_preemptive requires -thread flag
Docs: Using Lwt_preemptive requires -thread flag

Resolves #245.

[skip ci]

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