Add macOS specific steps to developer docs

Change-Id: I8517db6efb949ea9158405febf7b9faf14fd0f27
diff --git a/docs/developers/ b/docs/developers/
index cd21e90..b257189 100644
--- a/docs/developers/
+++ b/docs/developers/
@@ -20,4 +20,11 @@
 ## Mac-specific configuration
 On a Mac, you must also follow these steps:
-TODO(Issue #18): add macOS steps
+1. [Install Brew]( if not installed already.
+1. Use Brew to install the following packages: `bash`, `gnu-getopt`, and
+`gnu-sed`. Basically we want to get an updated bash version, an updated version
+of the `getopt` command, and the GNU version of `sed`.
+1. Set Brew's bash as the default shell (or just make sure to use Brew's bash
+when compiling the extension).
+1. Include the directories where `getopt` and `gnu-sed` were installed to