Add api-spec display

Add HTML exported from [editor.swagger.io](http://editor.swagger.io) or similar, for each example at [api-spec folder](https://github.com/ppKrauss/PostgREST-writeAPI/tree/master/api-spec), to display the API... But need to add `x-` properties in the description: see also [swagger-api/swagger-codegen](https://github.com/swagger-api/swagger-codegen), etc.

 