finalizing: tell people to backup gm9/out

also adds explanation of what each file is

_pages/en_US/finalizing-setup.txt

@@ -105,6 +105,11 @@ In this section, you will use a script to automate homebrew installation, SD car
 1. Follow the prompts in the script, answering each question that you are asked
   + If you encounter an error, follow the instructions in the error message or consult the [troubleshooting](troubleshooting#finalizing-setup) page
 1. Once the script says "Setup complete!", press (A) to power off the device
+1. Insert your SD card into your computer
+1. Copy the `/gm9/out/` folder to a safe location on your computer
+  + This folder contains critical file backups and should be backed up to multiple locations (i.e. cloud storage) if possible
+  + The two SysNAND files are your NAND backup and can be used to revert your console to a working state if it is bricked by a software issue
+  + The `essential.exefs` file contains your console's system-unique files and can be used to recover your data in the event of a hardware failure
 
 ___