Add readme file

Scripts/plasma-theme-switcher/README.md

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+# Automatically change Plasma theme between dark and light
+
+1. Place timer and service file in the respective user directory $HOME/.config/systemd/user
+2. Place the script into a directory and specify the path in the service file.
+3. Enable and start the timer:
+$ systemctl --user enable themer.timer
+$ systemctl --user start themer.timer