Added Stackage badges.

Author: svenpanne

README.md

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