Update README.md

Author: nopnop2002

stm32f407/README.md

@@ -20,10 +20,11 @@ In this example, you can select the CAN port.
 - CAN1 and CAN2   
 In this example, you can use CAN1 and CAN2 port.   
 
-
 # Remap CAN port
 
 STM32F405/407/415/417 have two CAN ports.   
+You can use only CAN1 or both CAN1 and CAN2, but you cannot use only CAN2.   
+
 These GPIOs can be used as CAN.   
 |CAN|RX|TX||
 |:-:|:-:|:-:|:-:|