Add Android Travis badge to README.md

Summary: Adds the Travis build badge for the android target to the README.md

Reviewed By: emilsjolander

Differential Revision: D4571693

fbshipit-source-id: a5110dac7d86989cc4a14b858c63d59332072869

Author: Robert Spencer

README.md

@@ -5,6 +5,7 @@
 [![iOS Status](https://badges.herokuapp.com/travis/facebook/yoga?env=TARGET=ios&label=iOS)](https://travis-ci.org/facebook/yoga)
 [![.NET Status](https://badges.herokuapp.com/travis/facebook/yoga?env=TARGET=net&label=.NET)](https://travis-ci.org/facebook/yoga)
 [![JavaScript Status](https://badges.herokuapp.com/travis/facebook/yoga?env=TARGET=js&label=JavaScript)](https://travis-ci.org/facebook/yoga)
+[![Android Status](https://badges.herokuapp.com/travis/facebook/yoga?env=TARGET=android&label=Android)](https://travis-ci.org/facebook/yoga)
 
 ## Building
 Yoga builds with [buck](https://buckbuild.com). Make sure you install buck before contributing to Yoga. Yoga's main implementation is in C, with bindings to supported languages and frameworks. When making changes to Yoga please ensure the changes are also propagated to these bindings when applicable.