Added doc reference about pwm frequency and resolution
#include <algorithm>
#ifdef ESP32
+// For info on frequency and resolution for ESP32, see
+// https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-reference/peripherals/ledc.html#ledc-api-supported-range-frequency-duty-resolution
#define CHANNEL_FREQUENCY 5000
#define CHANNEL_RESOLUTION 13
#define PWMRANGE 4095
