- Add explanation of direct-drive extruder vs. Bowden extruder

README.md

@@ -601,7 +601,12 @@ are needed to move an axis by 1 mm?
 
         e_steps_per_mm = (motor_steps_per_rev * driver_microstep) / (hob_effective_diameter * pi)
 
-See the example of [bowden
+The Lulzbots use a direct drive extruder, which is part of the
+extruder / hot end assembly. The Rostock uses a Bowden extruder, which
+affixed to the top of the Rostock, and is separated from the hot end
+by a long tube.  See both at [How To Calibrate Your
+Extruder](https://www.matterhackers.com/articles/how-to-calibrate-your-extruder).
+See the example of the [Bowden
 extruder](https://solidutopia.com/direct-drive-bowden-extruder/)
 
 Extruder with gear reduction: