Reference the map file in the specification (TAP4)

[vladimir-v-diaz]

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TAP 4 (the map file) describes how users may specify that a certain repository should be used for some targets, while other repositories are to be used for other targets.

We should reference the map file in the specification once TAP 4 is reviewed and accepted, which depends on the acceptance of TAP 7 (conformance testing). Pending pull request for the map file implementation is available here: https://github.com/theupdateframework/tuf/pull/430

A detailed outline of the map file should be covered in the specification, similar to pull request theupdateframework/tuf#440 that has now been merged.

[joshuagl]

The specification is now in a separate repository theupdateframework/specification, should this issue be moved there?

[lukpueh]

As discussed in the TUF community meeting on Dec 10, 2019 the changes of this PR require a minor bump of the spec version (see meeting notes about "Versioning" ).

[joshuagl]

I'm planning to take a stab at this over the next week or two.

[joshuagl]

For the interested, I submitted theupdateframework/taps#117 to try and clean up TAP 4 before I work on this.

[joshuagl]

I haven't forgotten this work, but I want to discuss and resolve https://github.com/theupdateframework/specification/issues/109 on documenting concepts first.