add twitter follow (apache#3104)

* add twitter follow

README.md

@@ -7,6 +7,7 @@
 [![Average time to resolve an issue](http://isitmaintained.com/badge/resolution/apache/incubator-dubbo.svg)](http://isitmaintained.com/project/apache/incubator-dubbo "Average time to resolve an issue")
 [![Percentage of issues still open](http://isitmaintained.com/badge/open/apache/incubator-dubbo.svg)](http://isitmaintained.com/project/apache/incubator-dubbo "Percentage of issues still open")
 [![Tweet](https://img.shields.io/twitter/url/http/shields.io.svg?style=social)](https://twitter.com/intent/tweet?text=Apache%20Dubbo%20(incubating)%20is%20a%20high-performance%2C%20java%20based%2C%20open%20source%20RPC%20framework.&url=http://dubbo.incubator.apache.org/&via=ApacheDubbo&hashtags=rpc,java,dubbo,micro-service)
+[![](https://img.shields.io/twitter/follow/ApacheDubbo.svg?label=Follow&style=social&logoWidth=0)](https://twitter.com/intent/follow?screen_name=ApacheDubbo)
 [![Gitter](https://badges.gitter.im/alibaba/dubbo.svg)](https://gitter.im/alibaba/dubbo?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge)
 
 Apache Dubbo (incubating) is a high-performance, Java based open source RPC framework. Please visit [official site](http://dubbo.incubator.apache.org) for quick start and documentations, as well as [Wiki](https://github.com/apache/incubator-dubbo/wiki) for news, FAQ, and release notes.