Add notes re YouTube playback, dual monitor setup.

Author: sakaki-

README.md

@@ -188,12 +188,16 @@ The whole process (from first power on to graphical desktop) should take less th
 
 The supplied image contains a fully-configured `~arm64` Gentoo system (*not* simply a [minimal install](https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Media#Minimal_installation_CD) or [stage 3](https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Media#What_are_stages_then.3F)), with a complete Portage tree already downloaded, so you can immediately perform `emerge` operations etc. Be aware that, as shipped, it uses **UK locale settings, keyboard mapping, timezone and WiFi regulatory domain**; however, these are easily changed if desired. See the Gentoo Handbook ([here](https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Base#Timezone) and [here](https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/System#Init_and_boot_configuration)) for details on the first three; to customize the WiFi regulatory domain, start the <kbd>Applications</kbd>&rarr;<kbd>Settings</kbd>&rarr;<kbd>RPi Config Tool</kbd> app, select the <kbd>WiFi</kbd> tab, choose your location from the drop-down, click <kbd>Save and Exit</kbd>, and reboot when prompted.
 
-> Please note that for non-UK keyboards, you will also need to edit the file `/etc/X11/xorg.conf.d/00-keyboard.conf` and replace  `gb` in the line `Option "XkbLayout" "gb"` with the appropriate code from [this list](https://pastebin.com/v2vCPHjs). Leave the rest of the file as-is, save, then reboot for the change to take effect. You will need to be the `root` user, or use `sudo` to edit this file (e.g., type `sudo mousepad /etc/X11/xorg.conf.d/00-keyboard.conf` in a terminal). 
+> Please note that for non-UK keyboards, you will also need to edit the file `/etc/X11/xorg.conf.d/00-keyboard.conf` and replace  `gb` in the line `Option "XkbLayout" "gb"` with the appropriate code from [this list](https://pastebin.com/v2vCPHjs). Leave the rest of the file as-is, save, then reboot for the change to take effect. You will need to be the `root` user, or use `sudo` to edit this file (e.g., type `sudo mousepad /etc/X11/xorg.conf.d/00-keyboard.conf` in a terminal).
+
+If you have an RPi4, and would like to configure a **dual monitor** setup, please see [these notes](https://github.com/sakaki-/gentoo-on-rpi-64bit/wiki/Set-Up-Dual-Displays-on-your-RPi4) (also contains some useful hints for single monitor deployment, applicable to all models).
 
 If you wish to add Bluetooth peripherals (e.g., a wireless mouse and/or keyboard) to your RPi, please see [below](#add_bt_device).
 
 If you would like to run **benchmarks** on your RPi system, please see [these notes](https://github.com/sakaki-/gentoo-on-rpi-64bit/wiki/Effective-Benchmarking-on-your-64-Bit-RPi-System).
 
+For hardware-accelerated YouTube playback, please see [this post](https://www.raspberrypi.org/forums/viewtopic.php?p=1589280#p1589280).
+
 The `@world` set of packages (from `/var/lib/portage/world`) pre-installed on the image may be viewed [here](https://github.com/sakaki-/gentoo-on-rpi-64bit/blob/master/reference/world-packages) (note that the version numbers shown in this list are Gentoo ebuilds, but they generally map 1-to-1 onto upstream package versions).
 > The *full* package list for the image (which includes @system and dependencies) may also be viewed, [here](https://github.com/sakaki-/gentoo-on-rpi-64bit/blob/master/reference/world-all-packages).