add a webchat freenode link to irc channel

Readme.md

@@ -36,7 +36,7 @@ $ npm install koa
  - [Guide](docs/guide.md)
  - [FAQ](docs/faq.md)
  - [中文文档](https://github.com/turingou/koa-guide)
- - __#koajs__ on freenode
+ - __[#koajs]__ on freenode
 
 ## Getting started
 
@@ -93,3 +93,4 @@ $ make test
 [coveralls-url]: https://coveralls.io/r/koajs/koa?branch=master
 [gitter-image]: https://badges.gitter.im/Join%20Chat.svg
 [gitter-url]: https://gitter.im/koajs/koa?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge
+[#koajs]: https://webchat.freenode.net/?channels=#koajs