Specify imagery requirements

[cholmes]

Maybe it goes in its own repo, but probably makes most sense in the 'spec'. But we need to say what are the requirements for an image to be in OIN. I forget exactly what we talked about in person, but it was something like:

• visual 3-band geotiff required
• CC-BY licensed

I think we talked about letting people put optional individual bands. That is also where processing tools could help, where we can process the json and also put it in the right geotiff format, possibly combining bands.

There's also questions about color curves - can people put any color curve they want on it? Does it have to be free of a color curve? Do we have a recommended (but not required) curve?