bltouch: Add initial support for a bltouch "extras" module

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

config/example-extras.cfg

@@ -43,6 +43,24 @@
 #   not run any special G-Code commands on deactivation.
 
 
+# BLTouch probe. One may define this section (instead of a probe
+# section) to enable a BLTouch probe. A virtual
+# "probe:z_virtual_endstop" pin is also created (see the "probe"
+# section above for the details).
+#[bltouch]
+#sensor_pin:
+#   Pin connected to the BLTouch sensor pin. This parameter must be
+#   provided.
+#control_pin:
+#   Pin connected to the BLTouch control pin. This parameter must be
+#   provided.
+#x_offset:
+#y_offset:
+#z_offset:
+#speed:
+#   See the "probe" section for information on these parameters.
+
+
 # Bed tilt compensation. One may define a [bed_tilt] config section to
 # enable move transformations that account for a tilted bed.
 #[bed_tilt]