Pivotal Tap

Previously, the CLI could be installed via the central homebrew repository.  Recently the restrictions placed on the formula in that became too onerous to continue hosting it there.  The formula is now hosted in the [Pivotal Tap][1].  This change updates the documentation with instructions on how to add the Pivotal Tap and install the CLI from it.

[1]: https://github.com/pivotal/homebrew-tap