Added Stackage badges.

Author: svenpanne

README.md

@@ -1 +1,4 @@
-[![Hackage](https://img.shields.io/hackage/v/GLURaw.svg)](https://hackage.haskell.org/package/GLURaw) [![Build Status](https://travis-ci.org/haskell-opengl/GLURaw.png?branch=master)](https://travis-ci.org/haskell-opengl/GLURaw)
+[![Hackage](https://img.shields.io/hackage/v/GLURaw.svg)](https://hackage.haskell.org/package/GLURaw)
+[![Stackage LTS](https://www.stackage.org/package/GLURaw/badge/lts)](https://www.stackage.org/lts/package/GLURaw)
+[![Stackage nightly](https://www.stackage.org/package/GLURaw/badge/nightly)](https://www.stackage.org/nightly/package/GLURaw)
+[![Build Status](https://img.shields.io/travis/haskell-opengl/GLURaw/master.svg)](https://travis-ci.org/haskell-opengl/GLURaw)