Add initial documentation for specialist-frontend

with @evilstreak @bestie @alicebartlett @themitchell @kalleth
Merge pull request #51 from alphagov/add-documentation

Add initial documentation for specialist-frontend
Fix screenshot url to use master branch
Merge pull request #52 from alphagov/add-documentation

Fix screenshot url to use master branch
Be explicit about document_presenter method
Add examples of pages rendered by the app
Tidy up formatting of README

Make it easier to read in text format.
Merge pull request #53 from alphagov/clean-up-readme

Clean up readme