Update readme to highlight interaction between "Authenticated" user group and Permission checking

[fchebbo]

This pull request adds a minor update to the readme.

The reason I am adding this, is that recently we ran into a issue where we were suddenly seeing the

Unable to find project for artifact copy: YOUR_PROJECT_WITH_ARTIFACTS
This may be due to incorrect project name or permission settings; see help for project name in job configuration.
Build step 'Copy artifacts from another project' marked build as failure

message after a Jenkins Update and couldn't figure out why. It was only after looking at the source code (in particular here) that I was able to determine the issue.

This PR is a suggestion to include this additional line in the readme, as I don't believe that one should need to look into the source code to determine such a behavior.

Testing done

Ensured the readme looks "good" on the github page. This PR contains no actual code change

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[MarkEWaite]

Thanks very much! Alternate phrasing that is optional. Change is accepted whether you take the suggestion or not.