Update README.md

[aarons]

Simple update that shows the path to Factorio on a Mac (for Steam users)

[R-O-C-K-E-T]

As far as I'm aware a Steam library can be in any directory and I think that pointing out a potentially incorrect directory could cause confusion.
Through Steam there is an option to 'Browse local files', which is a cross-platform way of finding the Factorio install.

However you could update the guessing logic in fetch.py/fetch_assets for automatically finding a Factorio install directory. The install docs could probably also be updated to recommend trying the guess first.

[aarons]

Ah, good idea! I didn't know steam would use different directories.

I'll close this for now.