Add notes about PWM freq making current readings unreliable.

- Added notes about how the PWM frequency can make the current readings unreliable. (from parent library)

README.md

@@ -162,7 +162,26 @@ Version History
 -   1.0.1 (2011-11-07): Adds support for Arduinos not based on ATmega168/328.
 -   1.0.0 (2011-10-28): Original release.
 
-Notes about timers:
+### Current readings
+-------------------
+
+The current readings returned by `getM1CurrentMilliamps` and
+`getM2CurrentMilliamps` will be noisy and unreliable if you are using
+a PWM frequency below about 5 kHz.  We expect these readings to
+work fine if you are using a board based on the ATmega168, ATmega328P,
+or ATmega32U4, since this library uses 20 kHz hardware PWM on
+those boards.
+
+On other boards, this library uses `analogWrite` to generate PWM
+signals, which usually means that the PWM frequency will be too low to
+get reliable current measurements.  If `analogWrite` uses a frequency
+of 490 Hz or more on your board, you can add a 1 µF
+(or larger) capacitor between each current sense line you are using
+and GND.  To make `getM1CurrentMilliamps` work well, you would add the
+capacitor between M1CS and GND.  To make `getM2CurrentMilliamps` work
+well, you would add the capacitor between M2CS and GND.
+
+### Notes about timers:
 -------------------
 
 -   Timer0 is used by the functions millis(), delay(), micros() and delayMicroseconds(),