Add bullet point to Before You Begin section in system-firmware doc (#1134)

Co-authored-by: Jacob Kauffmann <jacobgkau@users.noreply.github.com>

Author: nonergodic

content/system-firmware.md

@@ -24,9 +24,11 @@ These instructions are for System76 owners who have been prompted for a firmware
   * USB Type-C charging is **not** supported during firmware updates on the following models: Galago Pro (galp3-c, galp4), Darter Pro (darp5, darp6)
 * **Disconnect external devices**, including any displays or USB devices.
   * If external devices are connected, the firmware update may restart without being written (and without showing an error).
+* Make sure you can **see the System76 splash screen on boot**.
+  * Some GPU ports might not provide a signal before booting is complete. You may need to switch from using one type of cable to another, or use the built-in screen on a laptop.
 * **Take a picture of these instructions** or pull up [s76.co/sfu](https://s76.co/sfu) on a phone or other device for reference.
 * During the firmware updating process, **the system will restart several times**. Prompts will occasionally ask for user involvement.
-  
+
 ### Starting the Update (GUI)
 
 In Pop!\_OS, you can check for firmware updates using **Settings -> Firmware**.