Write repo spec

[edsko]

This should document:

• [x] Repository layout -- See RepoLayout and co
• [ ] Order in which files are to be changed and updated
• [ ] Which proxy hints should be provided (esp for 'dumb' file mirrors)
• [ ] Specification of the index tarball (for instance, see https://github.com/commercialhaskell/all-cabal-files/issues/8).