Add codecov.io badge to README.md. (#1116)

Author: ajsecord

README.md

@@ -3,6 +3,7 @@
 <img align="right" src="mdc_hero.png" width="300px">
 
 [![Build Status](https://travis-ci.org/material-components/material-components-ios.svg?branch=develop)](https://travis-ci.org/material-components/material-components-ios)
+[![Code coverage](https://img.shields.io/codecov/c/github/material-components/material-components-ios/develop.svg)](https://codecov.io/gh/material-components/material-components-ios/branch/develop)
 
 Material Components for iOS (MDC-iOS) helps developers execute [Material Design](https://www.material.io). Developed by a core team of engineers and UX designers at Google, these components enable a reliable development workflow to build beautiful and functional iOS apps. Learn more about how Material Design supports design and usability best practices across platforms in the  [Material Design Platform Adaptation guidelines](https://material.io/guidelines/platforms/platform-adaptation.html).