encoder_rp2.py: Add code comment re pullups.

encoders/encoder_rp2.py

@@ -21,7 +21,8 @@ import rp2
 # work: https://github.com/micropython/micropython/issues/8086
 # so using arrays.
 def make_isr(pos):
-    old_x = array('i', (0,))
+    old_x = array("i", (0,))
+
     @micropython.viper
     def isr(sm):
         i = ptr32(pos)
@@ -33,11 +34,14 @@ def make_isr(pos):
             s: int = 1 if (x ^ y) else -1
             i[0] = i[0] + (s if (x ^ p[0]) else (0 - s))
             p[0] = x
+
     return isr
 
+
 # Args:
 # StateMachine no. (0-7): each instance must have a different sm_no.
 # An initialised input Pin: this and the next pin are the encoder interface.
+# Pins must have pullups (internal or, preferably, low value 1KΩ to 3.3V).
 class Encoder:
     def __init__(self, sm_no, base_pin, scale=1):
         self.scale = scale