Modify README.md to explain that this code has been moved into the concurrent-ruby gem.

Author: keithrbennett

README.md

@@ -1,9 +1,13 @@
-# Threadsafe
+# Threadsafe (Inactive, code moved to concurrent-ruby gem and repo.)
 
 [![Gem Version](https://badge.fury.io/rb/thread_safe.svg)](http://badge.fury.io/rb/thread_safe) [![Build Status](https://travis-ci.org/ruby-concurrency/thread_safe.svg?branch=master)](https://travis-ci.org/ruby-concurrency/thread_safe) [![Coverage Status](https://img.shields.io/coveralls/ruby-concurrency/thread_safe/master.svg)](https://coveralls.io/r/ruby-concurrency/thread_safe) [![Code Climate](https://codeclimate.com/github/ruby-concurrency/thread_safe.svg)](https://codeclimate.com/github/ruby-concurrency/thread_safe) [![Dependency Status](https://gemnasium.com/ruby-concurrency/thread_safe.svg)](https://gemnasium.com/ruby-concurrency/thread_safe) [![License](https://img.shields.io/badge/license-apache-green.svg)](http://opensource.org/licenses/MIT) [![Gitter chat](http://img.shields.io/badge/gitter-join%20chat%20%E2%86%92-brightgreen.svg)](https://gitter.im/ruby-concurrency/concurrent-ruby)
 
 A collection of thread-safe versions of common core Ruby classes.
 
+__This code base is now part of the concurrent-ruby gem
+at https://github.com/ruby-concurrency/concurrent-ruby.
+The code in this repository is no longer maintained.__
+
 ## Installation
 
 Add this line to your application's Gemfile: