Cocoa Pods installation

I added instructions to the README.md for cocoa pods installation. I pulled the video and warning from the [sharekit](https://github.com/ShareKit/ShareKit/wiki/Installing-sharekit/076eb363be39668961b3f2c640855d181bc684ac) cocoa pods installation instructions. I figured this could save time over having to lookup the pod on [cocoapods.org](http://cocoapods.org).