Include more info about this repo in the README file.

README.md

@@ -2,4 +2,8 @@
 
 [![NetMQ NuGet version](https://img.shields.io/nuget/v/NetMQ.svg)](https://www.nuget.org/packages/NetMQ/) [![NetMQ NuGet prerelease version](https://img.shields.io/nuget/vpre/NetMQ.svg)](https://www.nuget.org/packages/NetMQ/)
 
-Release branch, for more information about NetMQ go to https://github.com/zeromq/NetMQ
+This repo tracks version 3.x of NetMQ, which has been superceeded by version 4.x.
+
+Raise issues related to version 3.x here. Important fixes are cherry picked from 4.x, though new features and major work is applied only to 4.x. Users are recommended to update to version 4.x.
+
+For more information about NetMQ go to https://github.com/zeromq/NetMQ