Add "no longer maintained" warning to README

Author: bajtos

Diff for: README.md

@@ -1,5 +1,7 @@
 # loopback-getting-started
 
+**⚠️ This LoopBack 3 example project is no longer maintained. Please refer to [LoopBack 4 Examples](https://loopback.io/doc/en/lb4/Examples.html) instead. ⚠️**
+
 This is the example application that accompanies the [Getting started with LoopBack](http://loopback.io/doc/en/lb3/Getting-started-with-LoopBack.html) tutorial. Follow the steps in the tutorial to create this application from scratch.
 
 NOTE: This repository has commits for each step of the tutorial, so you can pick up the tutorial at any point along the way: