[Docs] Update feature_split_keyboard.md (#13205)

docs/feature_split_keyboard.md

@@ -60,6 +60,7 @@ The 3 wires of the TRS/TRRS cable need to connect GND, VCC, and D0/D1/D2/D3 (aka
 The 4 wires of the TRRS cable need to connect GND, VCC, and SCL and SDA (aka PD0/pin 3 and PD1/pin 2, respectively) between the two Pro Micros. 
 
 The pull-up resistors may be placed on either half. If you wish to use the halves independently, it is also possible to use 4 resistors and have the pull-ups in both halves.
+Note that the total resistance for the connected system should be within spec at 2.2k-10kOhm, with an 'ideal' at 4.7kOhm, regardless of the placement and number.
 
 <img alt="sk-i2c-connection-mono" src="https://user-images.githubusercontent.com/2170248/92297182-92b98580-ef77-11ea-9d7d-d6033914af43.JPG" width="50%"/>