README: mention max_usb_current setting on Raspberry Pi

README.md

@@ -237,6 +237,8 @@ Tested on a Raspberry Pi Zero with a clean install of minimal Raspbian Stretch (
 	CONF_SWAPSIZE=1024  
 	sudo /etc/init.d/dphys-swapfile start  
 ```
+* If using an external hard disk without an external power supply, ensure it gets enough power to avoid hardware issues when syncing, by adding the line "max_usb_current=1" to /boot/config.txt
+
 * Clone monero and checkout most recent release version:
 ```
         git clone https://github.com/monero-project/monero.git