Updated readme to address potential issue with MacOs

[Niminem]

I banged my head against the wall for a couple of hours trying to get nim/choosenim/etc to work on my mac- I clearly saw ~/.nimble/bin in my path via echo $path but it wouldn't work. Upon further investigation, apparently I didn't have a .zshrc file in my home directory and needed to add the path there.

This was on a clean installation of mac.

It may or may not be common so I figured it's best to add notes to the README.