Update README.md

Author: Bexin3

README.md

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 # Fast-ADC-to-PWM-for-SAMD21-Arduino
-A .ino project for SAMD21 Arduinos tested on Arduino Zero that allows conversion of Analog values into PWM duty cycle, tested frequencies are 10-200000hz
+A .ino project for SAMD21 Arduinos tested on Arduino Zero that allows conversion of Analog values into PWM duty cycle, tested frequencies are 0.035-200000hz
 Newest version support m0 seeduino xiao and custom choosing of analog input and PWM output pins, as well as higher frequency range.