Consistently use brand-capitalistion of 'GitHub'

Apologies, this is OCD but it jars with me!
Add more explanatory text, links, standardise on 'gu:who'

Created the https://github.com/gu-who-demo-org/people repo as a public
example of what the repo will look like (obviously, normally it would
be private!).


How do we "gu:who?" "GU-WHO?" "GU:Who?"

Use the decorative 'gu:who?' form only if name is *standing alone*, AND can
be presented in italics. Otherwise just 'gu:who', as the question mark is
way too distracting.

Also, use 'organisation' in preference to 'org' to avoid confusion with
domain names, as @gidsg pointed out.
Merge pull request #17 from guardian/text-tweaks

Text tweaks