README.md: add "tell about your app"

Author: greenrobot

README.md

@@ -1,5 +1,10 @@
 <img width="466" src="https://raw.githubusercontent.com/objectbox/objectbox-swift/master/images/logo.png">
 
+Do you ♥️ using ObjectBox?
+==========================
+We want to [hear about your app](https://docs.google.com/forms/d/e/1FAIpQLScIYiOIThcq-AnDVoCvnZOMgxO4S-fBtDSFPQfWldJnhi2c7Q/viewform)!
+It will - literally - take just a minute, but help us a lot. Thank you!​ 🙏​
+
 ObjectBox Swift
 ===============
 [![Version](https://img.shields.io/cocoapods/v/ObjectBox.svg?style=flat)](#cocoapods)