Update parallel.rst
Update parallel.rst

Added information on how to configure ``mem_free`` as a consumable resource, as by default the ``mem_free`` option only checks available memory upon spawning the engines. If we had a host with, say, 16 slots and 60 GB of memory and each job required 16GB, SGE would still happily spawn 16 engines on the host if ``mem_free`` wasn't a consumable.
lbeltrame/bcbio-nextgen,gifford-lab/bcbio-nextgen,vladsaveliev/bcbio-nextgen,biocyberman/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,SciLifeLab/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,biocyberman/bcbio-nextgen,vladsaveliev/bcbio-nextgen,SciLifeLab/bcbio-nextgen,elkingtonmcb/bcbio-nextgen,chapmanb/bcbio-nextgen,hjanime/bcbio-nextgen,brainstorm/bcbio-nextgen,hjanime/bcbio-nextgen,verdurin/bcbio-nextgen,gifford-lab/bcbio-nextgen,hjanime/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,brainstorm/bcbio-nextgen,guillermo-carrasco/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,gifford-lab/bcbio-nextgen,verdurin/bcbio-nextgen,fw1121/bcbio-nextgen,brainstorm/bcbio-nextgen,a113n/bcbio-nextgen,lpantano/bcbio-nextgen,fw1121/bcbio-nextgen,guillermo-carrasco/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,lpantano/bcbio-nextgen,mjafin/bcbio-nextgen,guillermo-carrasco/bcbio-nextgen,lpantano/bcbio-nextgen,lbeltrame/bcbio-nextgen,elkingtonmcb/bcbio-nextgen,SciLifeLab/bcbio-nextgen,mjafin/bcbio-nextgen,vladsaveliev/bcbio-nextgen,biocyberman/bcbio-nextgen,mjafin/bcbio-nextgen,verdurin/bcbio-nextgen,chapmanb/bcbio-nextgen,elkingtonmcb/bcbio-nextgen,fw1121/bcbio-nextgen