Add basic MacOS install instructions

[jacobwhall]

This PR adds basic instructions for installing Firefox and geckodriver on MacOS.

• curl is used rather than wget because wget is not installed by default on MacOS.
• There is a note for users of ARM Macs that they will need to download a different build of geckodriver.
• I replaced "FireFox" with "Firefox" throughout README.md

These instructions suggest that MacOS users manually install Firefox using a downloaded .dmg. I think this is the easiest way, but it would be nice for it all to happen within the terminal. This might be possible be installing a .pkg using the command line (which is also available from the Mozilla FTP server), or by using a package manager like homebrew.

Finally, MacOS users will almost certainly have their Firefox binary in a different location, so we should probably move this value into config.ini. I'd be happy to push another commit with this change.

[sgoodm]

Looks good, going to merge and we can adjust after testing with MacOS users later