remarkable: add more detail to README about launcher (#6397)

platform/remarkable/README_remarkable.txt

@@ -13,12 +13,33 @@ with a USB cable.
    ssh root@<your-remarkable>
 
    unzip <koreader-package>.zip
+
+# Launching using the included launcher
+
+KOReader includes a very simple launcher called `button-listen` which starts
+KOReader when the middle button is held down for 3 or more seconds in the
+default UI (xochitl).
+
    cp -v koreader/*.service /etc/systemd/system/
    systemctl enable --now button-listen
 
-Hold down the middle button for 3 seconds to start koreader. To return to
-xochitl just exit koreader (swipe down from the top of the screen, select icon
-in the top right, Exit, Exit).
+To return to the default UI just exit KOReader (swipe down from the top of the
+screen, select icon in the top right, Exit, Exit).
 
 Some reMarkable software updates will wipe the new systemd units so you will have
-to run the last two steps again when that happens.
+to run the two install steps again when that happens.
+
+# Using a different launcher
+
+The command that the launcher needs to run is:
+
+   /home/root/koreader/koreader.sh
+
+Alternatively, if you want the default behaviour of KOReader exit starting the
+default UI you can run (once on installation):
+
+   cp -v koreader/koreader.service /etc/systemd/system/
+
+Then to start KOReader the launcher needs to run:
+
+   systemctl start koreader