Use appropriate file extension for license file

This file is not written in Markdown, so the previous `.md` file extension was incorrect. The options for an appropriate standardized file name are:

- `LICENSE.txt`
- `LICENSE`

I chose the former because the file is in text format and the `.txt` file extension clearly communicates this to humans
and machines both, making the information in the file more accessible than would be the case with the latter file name.
This is the standard license file name in Arduino tooling projects for this reason.

Author: per1234