Create README.md

[ashpande]

Unlike https://github.com/arduino/ArduinoCore-samd this repository did not have a README.md file, so I created one along the same lines. It can be updated later with more details about the contents of the folder in the repository.

[CLAassistant]

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[per1234]

A minimal readme was added in #413 simply to act as a container for the status badges of the CI workflows added by that PR.

This PR proposes a more detailed readme, and so is not necessarily superseded by the one added in #413