Set up auto-publish

[plehegar]

This still needs the proper link to the Group decision to publish but otherwise it should work.

Note that the secret has been encoded at
https://github.com/w3c/fingerprinting-guidance/settings/secrets/actions

[plehegar]

Fixes #46

[samuelweiler]

@npdoty and @sandandsnow, Do you recall if we've ever recorded a decision to allow auto-publishing? If we don't find one, I suggest we propose that on the email list and take silence as assent - no need to put it on a meeting agenda.

[npdoty]

Apologies, maybe I don't understand the autopublish distinction.

Here's the decision (at least, the most recent I could quickly find in my email) to publish as a group note:
https://lists.w3.org/Archives/Public/public-privacy/2019JanMar/0072.html