Document deploying gu:who to a fresh Heroku instance (issue #22)

Requested by Philip Gardner (@NoLogo) :

https://github.com/guardian/gu-who/issues/22

Have run through these steps to confirm they work, the instance is here:

https://gu-who-nologo.herokuapp.com/
Additional explanation around the people repo
Merge pull request #23 from guardian/heroku-deploy

Document deploying gu:who to a fresh Heroku instance (closes issue #22)