developer-setup.md: Document recommendation for installing Homebrew.

docs/developer-setup.md

@@ -4,6 +4,19 @@ This page describes the common process for setting up the common local developme
 ## Setup
+### Step 0: Install Homebrew \[Mac OS X only\]
+
+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
+    typeset -U path
+    path=($path /opt/homebrew/bin)
+    ```
+
 ### Step 1: Install 1password-cli v2
 In order to protect sensitive values, many GitLab projects rely on 1password for storing secrets. This may be either directly, or through internal tooling such as [`pmv`](https://gitlab.com/gitlab-com/gl-infra/pmv).