Update README.md for macOS local dev instructions

[hanzalahwaheed]

Faced some issues setting up Ruby for the first time on a mac. MAybe we can add some macOS instructions and a similar common issues section that I can update with the issues I faced.

A similar thing can be done for windows users as well.

[MakisH]

Hi @hanzalahwaheed,

building the website is indeed tricky and one of the many reasons we wanted to switch to another framework (#472). I typically use the Docker-based workflow already listed in the README.md.

Even though adding documentation for macOS/Windows could be useful (but difficult for us to verify), it would completely change once we change the framework.