[Library Manager] Please add STM32RTC and STM32LowPower STM32duino libraries

[fpistm]

Hi,
Please add to Library Manager :

• https://github.com/stm32duino/STM32LowPower
• https://github.com/stm32duino/STM32RTC

Thanks in advance

P.S.: all have a release

[cmaglie]

done!