Add guidance on abbreviating million/billion

We had a story recently to change our abbreviation for billion from 'b' to 'bn'. As with most things, I don't really mind what the specific outcome of this pull request is, I'd just like it to be documented. This should be signed off by somebody who knows what they're doing with content (ie not me).

This follows the Guardian style guide:

- http://www.theguardian.com/guardian-observer-style-guide-b
- http://www.theguardian.com/guardian-observer-style-guide-m