No crate-level doccomment

[vi]

It's not obvious what gridly does from the root doc page.

[Lucretiel]

Yeah, that's one of the many things I need to take care of before 1.0. The short version is that the grid module provides traits for describing grids in terms of boundaries and lookups, and the rest of the crate has supporting types like Location for indexing into a grid, Vector for doing arithmetic between locations, etc.