Add note about secrets

[tenhobi]

Add a note about using and creating secrets.

Fixes #23

[ad-m]

I would add a sentence about the fact that when accessing the repository from which the GitHub Actions event is triggered, it creates a token for us without any additional configuration. This solves most cases of using the our action and saves reading long documentation.

[simon300000]

Note: If we want the push to trigger the actions again or something else, you will need to create you own token, since the GitHub's token does not have permission to do anything else other than push.

[koppor]

That what @ad-m says. I would also replace "docs" by "GitHub help" to make it clear that the docs is not by this action, but a general one.

[ZPascal]

I'm closing the PR because I think the use of tokens, permissions, and secrets is in general already documented here and in the GitHub upstream documentation. Feel free to reopen it if needed.