Add context about automatically completed questions

As requested by a user, I've added a bit more context about what constitutes a dataset page that we can automatically complete from. I've also written up how to publish metadata in DCAT format [here](https://github.com/theodi/open-data-certificate/wiki/Marking-up-your-data-in-DCAT) and linked to that from the ODC page.

Here's what it looks like now:

![odi open data certificate 6](https://cloud.githubusercontent.com/assets/109774/3554924/aafe1960-0911-11e4-8507-20b4e27cd9bf.png)