Link to source metadata file?

[cholmes]

Lots of imagery has very extensive source metadata. We should have a field to link to it if it lives online somewhere (and possibly let people upload the metadata next to the image?). The json should be authoritative on its fields, but the metadata can be informative from the provider.

Good providers will have a web accessible canonical metadata to refer to. I'd say we should allow any metadata format - we're not trying to make all the additional stuff machine readable, just the core. But eventually we could consider a recommended extended metadata format.

An example of this is landsat, with https://s3-us-west-2.amazonaws.com/landsat-pds/L8/016/019/LC80160192015092LGN00/LC80160192015092LGN00_MTL.txt