Update README
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added description for custom marked options
DoomHammer/docco,ShawnHuang/docco-multidir,guigrpa/docco,robertmackenzie/docco,gvithellyna/docco,osharim/docco,jrvidal/docco,jrvidal/docco,Hellyna/docco,tbremer/docco,oponder/docco,jalleyne/docco,guigrpa/docco-extended,whitetrefoil/docco,mhkeller/docco,lbn/docco,guigrpa/docco-extended,oponder/docco,lobrien/docco,whitetrefoil/docco,osharim/docco,tbremer/docco,ShawnHuang/docco-multidir,gvithellyna/docco,MaxwellRebo/docco,MaxwellRebo/docco,jalleyne/docco,lobrien/docco,mhkeller/docco,lbn/docco,guigrpa/docco,DoomHammer/docco,robertmackenzie/docco,Hellyna/docco