(docs) readme - Change memory map + Add section on CPU start-up

Memory map:
- Change to a 5x5 display
- Remove pointer to keypad (I can't see why you would want to relocate it)
- Add placeholder for bank-switching flag
- Remove `$30` as initial value of IP - that's covered (differently) by the new CPU start-up section

readme.md

@@ -67,16 +67,19 @@ With single stepping + verbose debugging output:
           C = 0
     - (bitwise, `0000 = NZOC`)
 
-## Memory map
+## CPU start-up
 
-- `00-0F` - display (4x4)
-- `10-19` - reserved for future use
-- `20   ` - keypad - value of the most recent keypress
-- `21   ` - pointer to display memory
-- `22   ` - pointer to keypad memory
-- `23-2F` - reserved for future use / variable storage
-- `30   ` - initial value for IP
-- `30-FF` - free
+When starting up, the CPU executes a `JMP $FF`.
+
+Put differently: it starts executing instructions at the address contained in `$FF`.
+
+## Cardiograph memory map
+
+- `00-19` - display (5x5)
+- `1A   ` - pointer to display memory
+- `1B   ` - keypad: value of latest key pressed
+- `1C   ` - reserved for future use (bank switching flag)
+- `1D-FF` - free
 
 ## Peripherals