Update README.md

Document the fact that this project is unmaintained and that an active fork exists.

README.md

@@ -4,6 +4,8 @@ Thrifty
 ![Build status](https://github.com/microsoft/thrifty/workflows/Pre-merge%20checks/badge.svg) [![Android Arsenal](https://img.shields.io/badge/Android%20Arsenal-Thrifty-green.svg?style=true)](https://android-arsenal.com/details/1/3227)
 [![codecov](https://codecov.io/gh/Microsoft/thrifty/branch/master/graph/badge.svg)](https://codecov.io/gh/Microsoft/thrifty)
 
+# NOTE: This repository is [DEPRECATED](https://github.com/microsoft/thrifty/issues/540)
+## An active fork can be found at https://github.com/benjamin-bader/thrifty
 
 Thrifty is an implementation of the Apache Thrift software stack, which uses 1/4 of the method count taken by
 the Apache Thrift compiler, which makes it especially appealing for use on Android.