format `docdir` reference in README
format `docdir` reference in README

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