Added Startup Security Utility to README.

Explains how to use these utilities which is useful to know when
dealing with new hardware security features.

[ci skip]

README.md

@@ -31,6 +31,7 @@ the two projects are meant to be used:
   - [Setup](#setup)
   - [Usage](#usage)
     - [Boot Disk](#boot-disk)
+    - [Startup Security Utility](#startup-security-utility)
     - [Customization](#customization)
   - [Troubleshooting](#troubleshooting)
   - [Versioning](#versioning)
@@ -154,6 +155,32 @@ documentation (provided here for reference):
       4. Wait for the macOS installer to load from the recovery partition.
       5. Use the dialog options to launch Disk Utility, reinstall the system, etc.
 
+Depending on your security settings, you might need to use the Startup Security Utility before using
+the Boot Disk (see below).
+
+### [Startup Security Utility](https://support.apple.com/en-us/HT208198)
+
+With newer hardware, you should be running with the Apple T2 Security Chip (found via  → About This
+Mac → Overview → System Report → Controller). In order to boot your machine using the Boot Disk,
+you'll need to *temporarily* disable the default security settings as follows:
+
+- Turn on or restart your Mac, then press and hold `COMMAND + R` immediately after seeing the Apple
+  logo.
+- Select Utilities → Startup Security Utility from the main menu.
+- Click "Turn Off Firmware Password".
+- Select "Secure Boot: No Security".
+- Select "External Boot: Allow booting from external media".
+- Quit the utility and restart the machine.
+
+You'll now be able to boot your system with the Boot Disk (see above).
+
+After the new operating system has been installed via the Boot Disk, *ensure you return to the
+Startup Security Utility and re-enable the following settings*:
+
+- Click "Turn On Firmware Password".
+- Select "Secure Boot: Full Security".
+- Select "External Boot: Disallow booting from external or removable media".
+
 ### Customization
 
 All executable scripts can be found in the `bin` folder: