Add notice that the package is obsolete now.

Author: rakyll

README.md

@@ -2,6 +2,8 @@
 
 [![Build Status](https://travis-ci.org/rakyll/coop.png?branch=master)](https://travis-ci.org/rakyll/coop)
 
+Note: This package became obsolete. I started it when I was learning Go a couple of years ago. I see so many better ways to implement them all now, so don't keep using this package as an ultimate reference.
+
 coop contains some of the most common concurrent program flows I personally use in Go. I'm suggesting you to use this package as a snippets reference/cheat sheet instead of a library. The functionally provided in this package can be obtained in many different ways, and frankly with more performant implementations depending on the type of your problem.
 
 coop contains implementations for the following flows: