Add doc written by @prattpratt

Tweaked syntax. Added section on appium predicate helpers

@prattpratt If you click `Edit` on an [existing doc](https://github.com/appium/appium/blob/master/docs/en/android_coverage.md) then you can provide a title and content for a new doc. By doing this on the GitHub UI, the website will track that you're the author and give proper credit. After that a pull request can be submitted.

If you don't care about the credit then I can merge this version of the doc.

Fix #2639