Add a little comment explaining how keypad entry workds

src/cpu.py

@@ -8,6 +8,17 @@ from adafruit_display_text import label
 import keypad
 from digitalio import DigitalInOut, Direction, Pull
 
+# TO USE KEYPAD TO MANIPULATE DATA:
+#
+# A button: run/halt
+# B button: when halted, toggles address/data entry
+# Right button: when halted, single-steps
+#
+# Address entry: press the two digits for the address. It is entered immediately (there's no "enter" key)
+# Data entry: likewise. After you press the second digit, it will automatically go to the next address.
+
+
+
 # to list board features: print(board.__dir__)
 
 btna = DigitalInOut(board.BTNA)