Adds further installation instructions for OS X.

I'm currently maintaining gulp-rsvg, a small wrapper around this module, and got a message this week that there was a problem installing node-rsvg. I found the command I ran in this pull request solved the issue, so I'd like to add it here for anyone else who might be having trouble installing this plugin.

https://twitter.com/HerrWulf/status/433972435529236481