Improve CocoaPods quality estimate

[ty0002]

We have 61 points: https://cocoapods.org/pods/BetweenKit/quality. Our short README affects this adversely.

Here is an example of a pod with a high quality estimate: https://cocoapods.org/pods/AFNetworking/quality.

We can do the following to improve this metric:

• Add changelog
• Add Doxygen docs
• Better metadata around where our documentation lies
• Expand on README (background, quick start guide)

[ty0002]

We have 66 points. Looks like the main thing letting us down is our 'Minimal README'. I'm not sure what we can add here; perhaps a simplified version of Usage? Or an index of the Docs?