add a pre-release badge to readme

Author: zhengqmark

README.md

@@ -3,6 +3,7 @@
 [[README for Cray]](README.cray.md) | [[README for PDL]](README.pdl.md)
 
 [![Build Status](https://travis-ci.org/pdlfs/deltafs-umbrella.svg?branch=master)](https://travis-ci.org/pdlfs/deltafs-umbrella)
+[![GitHub (pre-)release](https://img.shields.io/github/release-pre/pdlfs/deltafs-umbrella.svg)](https://github.com/pdlfs/deltafs-umbrella/releases)
 [![License](https://img.shields.io/badge/license-New%20BSD-blue.svg)](LICENSE.txt)
 
 deltafs-umbrella