Create a common section for responses

Previously, the two sections differed in style and missed entries that were
included in the other.

We may want to revisit this if/when we have eg DELETE routes.
Merge pull request #10 from alphagov/restructure_responses_in_readme

Create a common section for responses
Add 'json-schema' gem for validation
ManualsController validates incoming JSON

The `json-schema` gem is used for validation.
Move manual schema to separate file
Validate incoming JSON in the SectionsController

- We don't need to specify `require 'json'` - the tests still pass
  without it.