Add links to README.md
Add links to README.md

I added links to several of the files mentioned in the README so that it's easy to access the mentioned files. I also turned the plaintext URL's into links so that they can be clicked on the [shopify.github.io](http://shopify.github.io/active_shipping/) page.
Shopify/active_shipping,Goldbely/active_shipping,johannboutet/active_shipping