Add CircleCI status badge to README.md (#666)

Author: rohandubal

README.md

@@ -2,6 +2,7 @@
 
 [![GitHub release](https://img.shields.io/github/release/aws-amplify/aws-sdk-android.svg)](https://github.com/aws-amplify/aws-sdk-android/releases)
 [![Maven Central](https://img.shields.io/maven-central/v/com.amazonaws/aws-android-sdk-pom.svg)](https://search.maven.org/search?q=a:aws-android-sdk-pom)
+[![CircleCI](https://circleci.com/gh/aws-amplify/aws-sdk-android.svg?style=svg)](https://circleci.com/gh/aws-amplify/aws-sdk-android)
 
 The AWS SDK for Android provides a library and documentation for developers to build connected mobile applications using AWS.
 
@@ -190,4 +191,4 @@ Amazon Web Services
 
 ## License
 
-See the **LICENSE** file for more info.
+See the **LICENSE** file for more info.