PDM.h sample rates enhancement

In my recent project, the available sample rates 16000 and 41667 were insufficient. As a result, I made a tweak to the code and documentation to support the available sample rates.

Available here → GitHub - victorromeo/ArduinoPDM: Arduino PDM enhancement

Sample rates are 16000, 20000, 25000, 31250, 33333, 40000, 41667, 50000, 62500.

In addition I’ve added more notes to the PDM documentation covering when you can call setGain and setBufferSize which are insufficient in the current documentation.