Make it slightly easier for new devs to get started.

Added a test that dies if the configuration is changed.

This should make it impossible (and remember this is not a challenge) for any developer with a conscience  to add a new property without thinking about what new developers will do when configuring the project.
Update README.md

Fixed typo
Update README.md

Making documentation a bit clearer
Merge branch 'master' of github.com:guardian/frontend into prime-property
Merge branch 'prime-property' of github.com:guardian/frontend into prime-property
Merge pull request #2485 from guardian/prime-property

Make it slightly easier for new devs to get started.