extruder_stepper: Initial support for synchronized extruder steppers

Signed-off-by: Simo Apell <simo.apell@live.fi>
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example-extras.cfg

@@ -706,6 +706,20 @@
 #position_max:
 #   See the example.cfg for the definition of the above parameters.
 
+# Support for additional steppers synchronized to the movement of an
+# extruder (one may define any number of sections with an
+# "extruder_stepper" prefix).
+#[extruder_stepper my_extra_stepper]
+#extruder: extruder
+#   The extruder this stepper is synchronized to. The default is
+#   "extruder".
+#step_pin:
+#dir_pin:
+#enable_pin:
+#step_distance:
+#   See the "extruder" section in example.cfg for the definition of
+#   the above parameters.
+
 # Manual steppers (one may define any number of sections with a
 # "manual_stepper" prefix). These are steppers that are controlled by
 # the MANUAL_STEPPER g-code command. For example: "MANUAL_STEPPER