Added peripheral notes

firmware/Peripherals.md

@@ -0,0 +1,16 @@
+## SAM D20 Peripherals usage
+
+| Type | Peripheral | Function | Notes
+| --- | --- | --- | ---
+|*GLCK*|
+||gclk0|main|
+|*TC*||
+||tc0|
+||tc1|
+||tc3|there was the txco on glck_io[3], but the pin voltages didn't work
+||tc5|pwm
+|*SERCOM*||
+||sercom0|spi flash
+||sercom1|ublox gps
+||sercom2|
+||sercom3|radio|currently bitbanged as required pin layout broken in sercom