Document publish api better

[jonaskello]

The "Publish API" specification could be broken into parts:

• Promaster Publish File Format (ppff)
• Promaster Publish Transfer Protocol (pptp)
• Promaster Publish File System (ppfs)

The ppff is a file format in json that has data for a file iteslf, but also references to other files. The file format does not care about the content of the data.

The pptp is a protocol to transfer files that follow ppff. It is agnostic to the data content of the file and how the filesystem is layed out, it only cares about the references one file has to other files.

The ppfs is a specification of exactly which files are expected and what content they should have. Eg. root.json is a the root and references productXX.json etc.

Some info about the dependencies between the standards:

• pptp does not know of ppfs and vice versa.
• ppff does not know of either pptp or ppfs.