update README on usage
update README on usage

GoeGaming/node-steam-tradeoffers,Alex7Kom/node-steam-tradeoffers,vejbuch/node-steam-tradeoffers,lpinca/node-steam-tradeoffers,nseckinoral/node-steam-tradeoffers,FelikZ/node-steam-tradeoffer-manager,LorienMan/node-steam-tradeoffers