ext: hal: stm32cube: Update README with former fixups

Commit "ext: hal: stm32f1x and stm32f4x: disable i2c HAL"
and "stm32cube: Fix warning when SPI LL API is compiled"
modified stm32cube HAL but this was not documented in
series README file, which makes it hazardous to update
stm32cube packages.
This commit corrects this oversight.
Additionally STM32Cube README file to provide this information
and how to use it

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>