Introduce podspec and CocoaPods instructions

I've only added @gfontenot as an author as I couldn't find @tonyd256's full name or email.

One of the authors should `pod trunk push Argo.podspec` for the instructions in the README to work. Or the instructions should read:

```ruby
pod 'Argo', :git => 'https://github.com/thoughtbot/Argo'
```