Added multiple ASIN lookup instructions to README

Just wanted to mention in the README that multiple ASINs can be looked up at once.

Also, in case you're curious, the second ASIN I added links to the book "Agile Web Development with Rails".
http://www.amazon.com/Agile-Development-Rails-Pragmatic-Programmers/dp/1934356549/ref=sr_1_1?ie=UTF8&qid=1360527054&sr=8-1&keywords=Agile+Web+Development+with+Rails