add travis build status to README

README.md

@@ -1,6 +1,8 @@
 go-metrics
 ==========
 
+![travis build status](https://travis-ci.org/rcrowley/go-metrics.svg?branch=master)
+
 Go port of Coda Hale's Metrics library: <https://github.com/dropwizard/metrics>.
 
 Documentation: <http://godoc.org/github.com/rcrowley/go-metrics>.