v0.11.0

docs: document Nix installation (#8)

* docs: document Nix installation

* docs: correct the type count and the Nix instructions

The mapping count said 59 in one place and "more than 50" in another, with
the sentence cut off. `rewire types` reports 74.

`nix profile install` is a deprecated alias for `add`. allowUnfree was
claimed for every route, but the packages output sets it for its own nixpkgs
instance and only the overlay needs it from the user.