STM32U5: README update

Author: jeromecoutant

targets/TARGET_STM/README.md

@@ -116,6 +116,7 @@ This table summarizes the STM32Cube versions currently used in Mbed OS master br
 | L1          |    1.10.2    | https://github.com/STMicroelectronics/STM32CubeL1 |
 | L4          |    1.17.0    | https://github.com/STMicroelectronics/STM32CubeL4 |
 | L5          |    1.4.0     | https://github.com/STMicroelectronics/STM32CubeL5 |
+| U5          |    1.0.0     | https://github.com/STMicroelectronics/STM32CubeU5 |
 | WB          |    1.11.1    | https://github.com/STMicroelectronics/STM32CubeWB |
 | WL          |    1.0.0     | https://github.com/STMicroelectronics/STM32CubeWL |
 
@@ -168,7 +169,7 @@ You can also check in https://github.com/ARMmbed/stm32customtargets
 STM32 root directory is https://github.com/ARMmbed/mbed-os/tree/master/targets/TARGET_STM
 
 This contains:
-- all STM32 families directories: F0, F1, F2, F3, F4, F7, G0, H7, L0, L1, L4, WB
+- all STM32 families directories: F0, F1, F2, F3, F4, F7, G0, G4, H7, L0, L1, L4, L5, U5, WB, WL
 - Mbed OS porting layer common for all
 
 Each STM32 family contains several "sub-families".