[readme] add: android arsenal badge

Author: tomasznajda

README.md

@@ -1,5 +1,5 @@
 # rx-recaptcha
-[ ![Download](https://api.bintray.com/packages/tomasznajda/rx-recaptcha/rx-recaptcha/images/download.svg?version=16.0.0.2) ](https://bintray.com/tomasznajda/rx-recaptcha/rx-recaptcha/16.0.0.2/link) [![CircleCI](https://circleci.com/gh/tomasznajda/rx-recaptcha.svg?style=svg)](https://circleci.com/gh/tomasznajda/rx-recaptcha)\
+[ ![Download](https://api.bintray.com/packages/tomasznajda/rx-recaptcha/rx-recaptcha/images/download.svg?version=16.0.0.2) ](https://bintray.com/tomasznajda/rx-recaptcha/rx-recaptcha/16.0.0.2/link) [![Android Arsenal](https://img.shields.io/badge/Android%20Arsenal-rx--reCAPTCHA-brightgreen.svg?style=flat)](https://android-arsenal.com/details/1/7348) [![CircleCI](https://circleci.com/gh/tomasznajda/rx-recaptcha.svg?style=svg)](https://circleci.com/gh/tomasznajda/rx-recaptcha)\
 An easy way to use [SafetyNet reCAPTCHA](https://developer.android.com/training/safetynet/recaptcha.html) with [RxJava2](https://github.com/ReactiveX/RxJava)