Add reference to accessURL

[apuignav]

When we discuss downloading files from the Grid, we should mention this is not necessary at all thanks to the root protocol and we can show how to do that

apuignav@lxplus062:~/Starterkit$ lb-run LHCbDirac/latest $SHELL
apuignav@lxplus062:~/Starterkit$  dirac-dms-lfn-accessURL /lhcb/MC/2016/ALLSTREAMS.DST/00062514/0000/00062514_00000002_7.AllStreams.dst --Protocol=ROOT

Using the following list of SEs: ['CNAF_MC-DST', 'IN2P3-ARCHIVE', 'LAL_MC-DST']
Failed :
    IN2P3-ARCHIVE : File not at SE
Successful :
    CNAF_MC-DST :
        /lhcb/MC/2016/ALLSTREAMS.DST/00062514/0000/00062514_00000002_7.AllStreams.dst : root://xrootd-lhcb.cr.cnaf.infn.it//storage/gpfs_lhcb/lhcb/disk/MC/2016/ALLSTREAMS.DST/00062514/0000/00062514_00000002_7.AllStreams.dst
    LAL_MC-DST :
        /lhcb/MC/2016/ALLSTREAMS.DST/00062514/0000/00062514_00000002_7.AllStreams.dst : root://grid05.lal.in2p3.fr:1094//dpm/lal.in2p3.fr/home/lhcb/MC/2016/ALLSTREAMS.DST/00062514/0000/00062514_00000002_7.AllStreams.dst

I will explain this in my lesson and update the text later (I don't have time now), but this can serve as a reminder.