DOC: add gitt icon to README.md

Author: jreback

README.md

@@ -1,6 +1,8 @@
 # pandas: powerful Python data analysis toolkit
 
 [![Build Status](https://travis-ci.org/pydata/pandas.svg?branch=master)](https://travis-ci.org/pydata/pandas)
+[![Join the chat at
+https://gitter.im/pydata/pandas](https://badges.gitter.im/Join%20Chat.svg)](https://gitter.im/pydata/pandas?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
 
 ## What is it