developer-setup.md: Explicitly mention the "install" step.

docs/developer-setup.md

@@ -9,6 +9,7 @@ This page describes the common process for setting up the common local developme
 Install the *Homebrew* package manager for Mac OS X.
 *   Head to https://github.com/Homebrew/brew/releases and download a recent release as a `.pkg` (installer package).
+*   Execute the installer to install Homebrew.
 *   Add the Homebrew `bin/` directory to your path, e.g. by adding the following to `~/.zshenv`:
     ```shell