Update macOS instructions for Chromium installation

[CorentinB]

Add chromium installation to the macOS instructions.

[nlevitt]

Most people have google chrome installed and brozzler should find it automatically. Don't want to make people think it's more trouble to install brozzler than it actually is.

[dvanduzer]

I'd be happy with a different phrasing for this. Just something along the lines of "you must have Chrome installed, or follow these alternate, footnoted instructions for homebrew."

Would've saved me some time. :)

(to be clear: I am interested in some language specifically in the flow of "here's how to manually install all these dependencies" section, next to the part where rethinkdb is mentioned with the brew or GUI option)