added blank css file to plain-markdown layout directory
added blank css file to plain-markdown layout directory

tbremer/docco,jrvidal/docco,robertmackenzie/docco,osharim/docco,whitetrefoil/docco,lobrien/docco,guigrpa/docco-extended,guigrpa/docco,MaxwellRebo/docco,MaxwellRebo/docco,jalleyne/docco,DoomHammer/docco,ShawnHuang/docco-multidir,ShawnHuang/docco-multidir,lbn/docco,DoomHammer/docco,Hellyna/docco,lobrien/docco,mhkeller/docco,jrvidal/docco,lbn/docco,guigrpa/docco,gvithellyna/docco,gvithellyna/docco,mhkeller/docco,robertmackenzie/docco,oponder/docco,tbremer/docco,osharim/docco,whitetrefoil/docco,jalleyne/docco,guigrpa/docco-extended,oponder/docco,Hellyna/docco