Add LICENSE.md file

[s-celles]

Hello,

It will much clearer to add a LICENSE.md file as exposed at https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/adding-a-license-to-a-repository

Kind regards

Sebastien

[s-celles]

I think it's a BSD 3-Clause "New" or "Revised" License
PR #2 sent
Similar PR for other repositories
ArduinoGetStarted/button#9
ArduinoGetStarted/buzzer#3
ArduinoGetStarted/output#1

[ArduinoGetStarted]

Dear @scls19fr,

Thank you for your great suggestion on all our libraries.
We will update it in the next release!

Best regards,