Convert rdoc to markdown for syntax highlight

Changed from:

![](https://dl.dropboxusercontent.com/u/5978869/image/20131111_175125.png)

to:

![](https://dl.dropboxusercontent.com/u/5978869/image/20131111_175139.png)

You can also see [from](https://github.com/fluent/fluent-logger-ruby/blob/e63a40efdd6f3f1808cb6ca13a44f25bc6b554f7/README.rdoc) and [to](https://github.com/r7kamura/fluent-logger-ruby/blob/fd59251497b5be24c42480513dfab9d401112c8a/README.md).