Update README to include instructions for MacOS

[reydanro]

Per title, I was able to run locally on MacOS Sequoia 15.7.9 by mostly following the Linux instructions with some tweaks to socat.

$ socat -d -d pty,raw,echo=0,link=/tmp/ttyGRBLsim "EXEC:'./grblHAL_sim -n -s step.out -b block.out',pty,raw,echo=0"
2026/09/07 18:40:02 socat[2953] N PTY is /dev/ttys003
2026/09/07 18:40:02 socat[2953] N forking off child, using pty for reading and writing
2026/09/07 18:40:02 socat[2953] N forked off child process 2955
2026/09/07 18:40:02 socat[2953] N forked off child process 2955
2026/09/07 18:40:02 socat[2953] N starting data transfer loop with FDs [5,5] and [7,7]
2026/09/07 18:40:02 socat[2955] N execvp'ing "./grblHAL_sim"

Verified the simulator is functioning by running gSender and connecting to it.