The net result of this is that cd'ing into the main source directory and
running:
$ nix-shell
will get you every tool needed to immediately do RISC-V development with
picorv32 or picosoc (assuming you're targeting ICE40 or ECP5).
The shell.nix file contains many comments on how to use the resulting
environment.
Signed-off-by: Austin Seipp <aseipp@pobox.com>