docs: add github action to readme

[terriko]

We're expecting to "release" the github action https://github.com/intel/cve-bin-tool-action/ sort of in conjunction with the 3.3 release so I'm making this issue to make sure we mention it as a way to run cve-bin-tool in our readme and advertise it with the release.

[terriko]

This was done a while ago and can be closed:
https://github.com/intel/cve-bin-tool#using-cve-binary-tool-in-github-actions