Doc request: Pinging the server for the job ids

It is pretty convenient to request the list of job ids when we do not remember them. The readme does not illustrate this option . That could be done when logging to the booked node showing what is going on when issuing the following command

./g5k log -u joelegrand 

Then showing the list of JOB ids before issuing the full command

./g5k log -u joelegrand -j 2342772