create swagger docs for the API

[mbjones]

Create Swagger API documentation for the API. Use design info from #3.

[mbjones]

See evolving API as we edit it on Swagger here: https://app.swaggerhub.com/apis/mbjones/metadig

[gothub]

Note that there are now 2 versions of the API doc, with the new version using the updated
OpenAPI 3.0 version (old was 'Swagger 2.0'). The new doc is

https://app.swaggerhub.com/apis/mbjones/metadig/2.0.0-a1-oas3

This newer version is required to describe http multipart request body sections for PUT and POST requests.

Note that the new editor version is in beta