Add branding guidelines

Fixes https://github.com/theodi/shared/issues/231

Pushed to staging. Ready to merge when Adrian says he's happy. Page will be at:

https://certificates.theodi.org/using-the-marks

Here's a preview too:

![odi open data certificate](https://cloud.githubusercontent.com/assets/109774/3157680/c44579f4-eaf8-11e3-941a-3b57890e8adc.png)