PID example

docs/examples/line-follower-pid.md

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+
+```blocks
+let lasterror = 0
+let D = 0
+let I = 0
+let P = 0
+let error = 0
+loops.forever(function () {
+    error = sensors.color3.light(LightIntensityMode.Reflected) - 35
+    P = error * 5
+    I = I + error * 0.01
+    D = (error - lasterror) * 0.2
+    motors.largeBC.steer(P + (I + D), 100)
+    lasterror = error
+})
+```