Add arduino-sht to Library Manager

[dhasenfratz]

Dear maintainers,

can you please add the arduino-sht library to the Library Manager?
https://github.com/Sensirion/arduino-sht

We support with the library a large set of our temperature and humidity sensors (www.sensirion.com).

Thanks.

[dhasenfratz]

@per1234 Thanks for your fix of the keywords.txt file. I merged your PR and created and new release.

[cmaglie]

Done!