update README to provide users with npm-based options for development
update README to provide users with npm-based options for development

snaheth/pileup.js,hammerlab/pileup.js,piotr-gawron/pileup.js,hammerlab/pileup.js,piotr-gawron/pileup.js,hammerlab/pileup.js,piotr-gawron/pileup.js,snaheth/pileup.js,snaheth/pileup.js,snaheth/pileup.js,piotr-gawron/pileup.js,hammerlab/pileup.js