Update README to accept fine-grained tokens

[pnacht]

GraphQL now works with fine-grained tokens. My understanding is that this was the blocker to adopting fine-grained tokens in the Scorecard Action.

I unfortunately did not manage to get fine-grained tokens to work for random public repos using the Scorecard CLI, so I think we're still stuck using classic PATs there (but please try to solve this yourself!)... but they do work when running the Scorecard Action with a fine-grained PAT (see this example and the successful workflow run).

Therefore, this PR updates the README.md to show how to set the token up. Are changes needed anywhere else (other than the starter workflow)?

The README contained quite a bit of information on private repos, which I don't believe applies to fine-grained tokens. However, I don't have a private repo with GitHub Advanced Security, so I haven't been able to test this. This should be tested by someone with access to such repos.

The README also describes issues with SAML SSO, but these do not apply to fine-grained tokens, which are authorized at creation. As such, these comments have been removed. (Though it may be worth testing this just to be sure!)

Before merging this PR, we should make sure that:

• Action works with fine-grained tokens on private repositories
• Action works with fine-grained tokens on repos with SAML SSO

[codecov]

Codecov Report

Merging #1175 (e143085) into main (8808ed2) will decrease coverage by 0.98%.
The diff coverage is n/a.

❗ Current head e143085 differs from pull request most recent head ec6db9c. Consider uploading reports for the commit ec6db9c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1175      +/-   ##
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- Coverage   64.68%   63.70%   -0.98%     
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  Files           4        4              
  Lines         269      248      -21     
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- Hits          174      158      -16     
+ Misses         80       75       -5     
  Partials       15       15              

see 1 file with indirect coverage changes