document cabal sdist (hackage archive) format

(Imported from [Trac #759](http://hackage.haskell.org/trac/hackage/ticket/759), reported by claus on 2010-11-04)

I often find myself writing example code that I'd like to distribute via cabal, but without further burdening hackage with not generally useful packages.

The source code has an undocumented(?) option --remote-repo that seems to serve that purpose, but there is little documentation, sometimes conflicting info, about how to put this to use.

Mailing list threads:

[http://www.haskell.org/pipermail/haskell-cafe/2010-November/085860.html](http://www.haskell.org/pipermail/haskell-cafe/2010-November/085860.html)

[http://www.haskell.org/pipermail/haskell-cafe/2010-November/085900.html](http://www.haskell.org/pipermail/haskell-cafe/2010-November/085900.html)

Repo layout descriptions (differs between hackage 1.0 and 2.0, which is hardcoded in cabal-install):

[http://hackage.haskell.org/trac/hackage/wiki/HackageDB](http://hackage.haskell.org/trac/hackage/wiki/HackageDB)

[http://hackage.haskell.org/trac/hackage/wiki/HackageDB/2.0/URIs](http://hackage.haskell.org/trac/hackage/wiki/HackageDB/2.0/URIs)

Related tickets: #758, comment:6:ticket:428