add travis build status to readme

Author: mourner

README.md

@@ -17,6 +17,8 @@ We're happy to meet new contributors.
 If you want to **get involved** with Leaflet development, check out the [contribution guide][contribute].
 Let's make the best open-source library for maps that can possibly exist!
 
+[![Build Status](https://travis-ci.org/Leaflet/Leaflet.png)](https://travis-ci.org/Leaflet/Leaflet)
+
  [Vladimir Agafonkin]: http://agafonkin.com/en
  [contributors]: https://github.com/Leaflet/Leaflet/graphs/contributors
  [features]: http://leafletjs.com/features.html