Document how to provide GitHub credentials to PolicyPanda

[yrodiere]

Because it can be hard if you don't know Hub4j's GitHub API :)

Unfortunately the website is down (hub4j/github-api#2008), but the internet archive has something, so I also added alternate links.

voting group: @commonhaus/cf-egc

Do one of the following:

• Approve the PR or react with 👍 (:+1:) if it looks good to you
• Review with Comments or react with 👀 (:eyes:) if you're "ok" with it (it may not be your favorite)
• If you think it needs discussion or revision
  • Create a review, add your comments and require changes
  • Use the +- button to make a suggestion (instead of just adding a comment).