Add Amazon link

README.md

@@ -2,6 +2,8 @@
 
 [![Get it on Google Play](https://developer.android.com/images/brand/en_generic_rgb_wo_45.png)](https://play.google.com/store/apps/details?id=org.tasks)
 
+[![Get it on Amazon App Store](https://images-na.ssl-images-amazon.com/images/G/01/AmazonMobileApps/amazon-apps-store-us-black.png)](https://www.amazon.com/gp/product/B00QHGTL7O/ref=mas_pm_tasks_astrid_to_do_list_clone)
+
 [Get it on F-Droid](https://f-droid.org/repository/browse/?fdid=org.tasks)
 
 [![Build Status](https://travis-ci.org/abaker/tasks.svg?branch=master)](https://travis-ci.org/abaker/tasks)