Add default.nix

Add Nix expression (default.nix) to setup development environment for
fgui with the Nix package manager. With Nix installed, just run
"nix-shell" to get an interactive deveopment shell with needed
dependencies.

The same expression can also be used to fully build fgui:

  nix-build -o ../result

(-o ../result is used to prevent the result symlink from feeding back
into the source if/when doing a second build.)
README.asciidoc: (tiny) build instructions refactoring