Added Filament Sensor to the README

Added some background on the filament sensor to explain it better.

README.md

@@ -53,6 +53,7 @@ Features:
 *   Automatic operation of extruder/cold-end cooling fans based on nozzle temperature
 *   RC Servo Support, specify angle or duration for continuous rotation servos.
 *   Bed Auto Leveling.
+*   Support for a filament diameter sensor, which adjusts extrusion volume
 
 The default baudrate is 250000. This baudrate has less jitter and hence errors than the usual 115200 baud, but is less supported by drivers and host-environments.
 
@@ -391,6 +392,13 @@ For example, suppose you measured the endstop position and it was 20mm to the ri
 
 That's it.. enjoy never having to calibrate your Z endstop neither leveling your bed by hand anymore ;-)
 
+Filament Sensor
+---------------
+Supports the use of a real time filament diameter sensor that measures the diameter of the filament going into the extruder and then adjusts the extrusion rate to compensate for filament that does not match what is defined in the g-code.  The diameter can also be displayed on the LCD screen. This potentially eliminates the need to measure filament diameter when changing spools of filament. Gcode becomes independent of the filament diameter. Can also compensate for changing diameter.
 
+For examples of these sensors, see: http://www.thingiverse.com/thing:454584, https://www.youmagine.com/designs/filament-diameter-sensor, http://diy3dprinting.blogspot.com/2014/01/diy-filament-diameter-sensor.html. Any sensor which produces a voltage equivalent to the diameter in mm (i.e. 1v = 1mm) can be used. This provides a very simple interface and may encourage more innovation in this area.
 
+4 new Mcodes are defined to set relevant parameters: M404, M405, M406, M407 - see above.
+
+ Implements a delay buffer to handle the transit delay between where the filament is measured and when it gets to the extruder.