add docs: Installing Locust on Mac OS

[yurtaev]

No description provided.

[Jahaja]

Thanks!

[heyman]

I'm not sure if installing gevent using Homebrew should be the recommended way of installing gevent, when there is pip and virtualenv. Did you have problems installing gevent using just pip? In that case I think it could be resolved by just installing libevent using Homebrew (or another package manager for that matter).

[cgbystrom]

I'm not sure if installing gevent using Homebrew should be the recommended way of installing gevent

Agree. Homebrew for libevent or libuv is probably a good idea but pip is probably better at managing the Python dependencies.

[cgbystrom]

I took the liberty of patching the docs.

@yurtaev Thanks for submitting!