updated usage description in docs
updated usage description in docs

Winterflower/khmer,jas14/khmer,kdmurray91/khmer,ged-lab/khmer,kdmurray91/khmer,F1000Research/khmer,Winterflower/khmer,souravsingh/khmer,F1000Research/khmer,kdmurray91/khmer,ged-lab/khmer,F1000Research/khmer,souravsingh/khmer,jas14/khmer,jas14/khmer,ged-lab/khmer,Winterflower/khmer,souravsingh/khmer