Update the doc about building and running on macOS

[lkuty]

On macOS M2, I had to point to the directory containing the CBQN library to build and run beacon. The directory where CBQN was installed on my system is /Users/ludo/Downloads/github/CBQN. Could you update the README ?

Building and running:

export DYLD_FALLBACK_LIBRARY_PATH=/Users/ludo/Downloads/github/CBQN
RUSTFLAGS='-L /Users/ludo/Downloads/github/CBQN' cargo run --release