enable anchorlink for header

Header can be linked to directly. Here's a snapshot when i applied this to docker's current docs

![screenshot 2014-07-20 00 32 42](https://cloud.githubusercontent.com/assets/62707/3635197/af1894f6-0f62-11e4-8974-6877931011d7.png)

I guess that if we apply this, docker's style will need to be updated too to make it look nicer :smile: