Elaborate on project goals and package structure in README.md

[imalsogreg]

Could you give some more info - both in terms of API boundary and specific examples - of what hnix-store is meant to do? I've heard you say that we want to communicate with the nix daemon, but I'm not sure if that is the high level goal of the project, a design constraint for the whole project, or a design constraint for a subset of the packages within the project.

The ideal description would target people with only a vague understanding of the nix daemon and its relationship to nix-store, nix-instantiate and nix-env - mainly as an aid to contributors who know more about Haskell than Nix, like yours truly. :)