Fixed broken link and updated Usage
Fixed broken link and updated Usage

This link https://github.com/spree-contrib/contact_us points to no where. It has been removed now.
Also a little heads-up about using the gem and a link to a recommended guide has been added.
JDutil/spree_contact_us,JDutil/spree_contact_us,spree-contrib/spree_contact_us,JDutil/spree_contact_us,spree-contrib/spree_contact_us