Add hackage link

README.md

 # DateTime
 
-[![Build Status](https://travis-ci.org/stackbuilders/datetime.svg?branch=master)](https://travis-ci.org/stackbuilders/datetime)
+[![Build Status](https://travis-ci.org/stackbuilders/datetime.svg?branch=master)](https://travis-ci.org/stackbuilders/datetime) [![Hackage version](https://img.shields.io/hackage/v/datetime-sb.svg)](http://hackage.haskell.org/package/datetime-sb)
 
 The original DateTime package on Hackage hasn't been updated in a long
 time, and it won't build on GHC 7.10. This fork by Stack Builders, on