Add demonstration of using GitHub Actions to execute Arduino CI

[ianfixes]

Set up GitHub Actions for this repository to demonstrate how a user of arduino_ci would set it up for themselves

Related to #199 but this concerns how an end user (not a contributor to arduino_ci itself) would leverage this tool

[jgfoster]

Should we choose a project like Blink and make it the "poster child" for how the author of an external library would leverage arduino_ci? If so, then this issue would be placed there and the local README.md would point to that project? (And, perhaps, now that I know a bit more I should re-record the video describing it!?)

[ianfixes]

Yes... this might be a sub-task of #164 ... ditto for #77

[prestoncarman]

Following the example in #77 and the README example configs, I set up automated testing for these vendors in the Blink repository. The change is in PR. The update includes a table on the README showing all the Vendors and Operating systems that are used in the sample project.

Notes: macOS test is dependent on #199.

[jgfoster]

You can get macOS to pass now by explicitly downloading a more recent version of Arduino.app in your Action script. See what was added to Blink.

[ianfixes]

I really like this, I might not get to it today but I will soon

[ianfixes]

I think this is completed, and we have our own action now https://github.com/marketplace/actions/arduino_ci