Update README.md

Installation information for Mac users (RustAudio#130)

README.md

@@ -19,6 +19,8 @@ portaudio = "X.Y.Z"
 
 **rust-portaudio** will try to detect portaudio on your system and, failing that (or if given the `PORTAUDIO_ONLY_STATIC` environment variable on the build process), will download and build portaudio statically. If this fails please let us know! In the mean-time, you can manually [download and install PortAudio](http://www.portaudio.com/download.html) yourself.
 
+On Mac OS X, you may need to install manually `portaudio` and `pkg-config` (using [brew](http://brew.sh/), run `brew install portaudio` and `brew install pkg-config`)
+
 **rust-portaudio** is built using cargo, so just type `cargo build` at the root of the **rust-portaudio** repository.
 
 You can build the tests and examples with `cargo test`, and the documentation with `cargo doc`.