setup-guide.rst: note amaranth-yosys requirement
setup-guide.rst: note amaranth-yosys requirement

Notes that amaranth-yosys may be required.

Installation of amaranth-yosys needs pip3 which is usually supplied by
conda or a Python virtual environement.

Signed-off-by: Alan Green <97e77871b5fcc4941bebceb4338c97c1fd149f8c@google.com>

google/CFU-Playground,google/CFU-Playground,google/CFU-Playground,google/CFU-Playground