Add brief note on PAT documentation

[kfcampbell]

This small PR adds a note on what types of authentication are supported by the SDK.

Nick: do you think I should take out the note about future work planned for GitHub Apps?

[what-the-diff]

PR Summary

• Enhanced Documentation
  This update improves the project documentation files, specifically, the README.md file. Now, it contains comprehensive information about the 'authentication' aspect of the Software Development Kit (SDK). This new addition details on the various types of 'personal access tokens' that are supported by this SDK. Moreover, it also provides insights into future support plans for 'GitHub Apps authentication'. This improves user understandability and assists them in better utilizing the SDK functionalities.

[github-actions]

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