Fix broken link to grape-entity documentation
Merge pull request #474 from MichaelXavier/patch-1

Fix broken link to grape-entity documentation
Update README.md

Proper name for error response shortcut method is #error_response, 
not #rack_response.
Merge pull request #476 from markiz/patch-1

Update README.md
Added support for the :jsonapi, application/vnd.api+json media type registered at http://jsonapi.org.
Added param validator for an allowed list of values.
Fixed default_error_formatter which takes a format symbol.