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.

[janAkali]

I clearly saw ~/.nimble/bin in my path via echo $path but it wouldn't work.

$path and $PATH are two different things. $PATH is widely used by everything, but I've never seen $path until today.

[Niminem]

@Archargelod Interesting. Oddly enough, both $PATH and $path shown the same for me and figured it was just case-insensitive and the same things. I've accepted those changes from your end. I think we may be good here to finish this one out :)

[Niminem]

@Archargelod @dom96 Gentlemen, I would love to make some Youtube videos about Nim and simply point them to Choosenim for installation. Because many of the viewers will not be from a technical background (I do SEO professionally), many will have a Mac and may encounter the issue described above.

Can we merge this PR asap?

[Niminem]

@dom96 ?