Include iOS README in binary pod

[1ec5]

This PR adds the iOS-specific README to the build folder that gets zipped up and distributed as a CocoaPods binary pod. Without this README, CocoaPods dings the MapboxGL pod big time in its quality metrics for lacking a README. We’ve been compiling this README to serve as the index page of the API documentation that gets generated, but a README also needs to set right alongside the library and resource bundle.

/cc @incanus @friedbunny

[friedbunny]

👍

https://guides.cocoapods.org/making/quality-indexes.html

README Scoring

The README score is based on an algorithm that looks at the variety of the bundled README. You can run the algorithm against any URL here on scoreme.report. A README is the front-page of your library, it can provide an overview of API or show what the library can do.

Strange as it sounds, if you are providing a binary CocoaPod, it is worth embedding your README.md inside the zip. This means CocoaPods can use it to generate your Pod page.

[incanus]

Great work — just noticed this ranking business recently myself. Good discussion on the mailing list at https://groups.google.com/forum/#!topic/cocoapods/Pvjsd57zKAk as well.