Add build status icon to README

Author: Zachary Scott

README.rdoc

@@ -3,6 +3,7 @@
 home :: https://github.com/rdoc/rdoc
 rdoc :: http://docs.seattlerb.org/rdoc
 bugs :: https://github.com/rdoc/rdoc/issues
+build status :: {<img src="https://travis-ci.org/rdoc/rdoc.svg?branch=master" alt="Build Status" />}[https://travis-ci.org/rdoc/rdoc]
 code quality :: {<img src="https://codeclimate.com/badge.png" alt="code climate">}[https://codeclimate.com/github/rdoc/rdoc]
 
 == Description