Updating README with info on OS X 10.8/rbenv

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Hey, I just found a problem today while using rbenv on OS X 10.8 Mountain Lion that I thought should be in the docs. Thanks!

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I referenced the following on on the readme.md update:

• http://overwatering.org/blog/2012/08/setting-path-osx-mountain-lion/
• http://stackoverflow.com/questions/135688/setting-environment-variables-in-os-x