README.md: add suggestion for calibrating during startup

Author: merge

README.md

@@ -26,6 +26,28 @@ To _uninstall_, again go inside the extracted directory, and do
 ### multitouch
 if you use tslib 1.10 or higher (libts.so.0.7.0 or higher), you have multitouch.
 
+### calibrate (if not calibrated) before X starts the display
+(untested :) If you use the `linear` module in `ts.conf` you want to calibrate
+your screen, using `ts_calibrate`. To do this once in case not already done, you'd
+add something like the following snippet to your display manager's init script
+* `/etc/gdm3/Init/Default` for Gnome3's gdm
+* `/etc/X11/xdm/Xsetup` for X's display manager
+* `/etc/kde4/kdm/Xsetup` for KDE's kdm
+and so on:
+
+		TSCALIB_BIN=/usr/bin/ts_calibrate
+
+		# calibrate touchscreen, if no calibfile exists
+		if ! [ -f "${TSLIB_CALIBFILE}" ] ; then
+			if ! ${TSCALIB_BIN} ; then
+				echo "calibration failed!"
+			else
+				echo "calibration done."
+			fi
+		fi
+
+To trigger re-calibration, simply delete the `TSLIB_CALIBFILE` file and reboot.
+
 ### contact
 * If you have problems or suggestions, feel free to
 [open an issue](https://github.com/merge/xf86-input-tslib/issues).