Update README.md

Author: avandalen

README.md

@@ -1,7 +1,9 @@
 ### avdweb_AnalogReadFast
 The standard Arduino analogRead() takes about 112us for the AVR and 425us for the SAMD21, this is very slow. 
 The new function analogReadFast() takes just 21us. 
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 Compatible with
 - AVR boards: Arduino Uno, Nano, Mega, Leonardo, etc.
 - SAMD21 boards: Arduino Zero, SAM 15x15, etc. 
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 http://www.avdweb.nl/arduino/libraries/fast-10-bit-adc.html