skew_correction: add documentation

Signed-Off-By:  Eric Callahan <arksine.code@gmail.com>

config/example-extras.cfg

@@ -377,6 +377,28 @@
 #   if retries are enabled then retry if largest and smallest probed points
 #   differ more than retry_tolerance
 
+# Printer Skew Correction.  It is possible to use software to correct
+# printer skew across 3 planes, xy, xz, yz.  This is done by printing
+# a calibration model along a plane and measuring three lengths.  See
+# skew_correction.md in the docs directory for more details.
+#
+#[skew_correction]
+#xy_ac_length:
+#xy_bd_length:
+#xy_ad_length:
+#   The measured lengths (in mm) on the xy plane skew calibration print.
+#   If one length is provided all three lengths must be provided.  The default
+#   is no measurement, which results in no skew correction along the xy plane.
+#xz_ac_length:
+#xz_bd_length:
+#xz_ad_length:
+#   The same as above, on the xz plane.  The default is no skew correction
+#   on the xz plane.
+#yz_ac_length:
+#yz_bd_length:
+#yz_ad_length:
+#   The same as above on the yz plane.  The default is no skew correction
+#   on the yz plane.
 
 # In a multi-extruder printer add an additional extruder section for
 # each additional extruder. The additional extruder sections should be