add badges

README.md

@@ -1,5 +1,7 @@
 # go-colorable
 
+[![Build Status](https://travis-ci.org/mattn/go-colorable.svg?branch=master)](https://travis-ci.org/mattn/go-colorable) [![Coverage Status](https://coveralls.io/repos/github/mattn/go-colorable/badge.svg?branch=master)](https://coveralls.io/github/mattn/go-colorable?branch=master) [![Go Report Card](https://goreportcard.com/badge/mattn/go-colorable)](https://goreportcard.com/report/mattn/go-colorable)
+
 Colorable writer for windows.
 
 For example, most of logger packages doesn't show colors on windows. (I know we can do it with ansicon. But I don't want.)