Install instructions for pi

[Michael Latiolais]

I was helping someone install .57 on a pi4, and the instructions on the mpf page results in an error when installing mpf-mc. It looked like cython wasn't able to find some files in mpfmc/core/audio

I just did a regular pip install and it worked:

pip install mpf-mc==0.57.1 (I think,  I'm reporting this from memory).