Add a Contributor Agreement

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes #xyz
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)

What kind of change does this PR introduce?

• Adds a contributor licensing agreement to satisfy conditions of the Artistic License v2.0
• Updates the CONTRIBUTING.md file to provide relevant information (removing lorem ipsum stand-in text)
• Adds a warning for first-time contributors to agree to the contributor licensing agreement

Does this PR introduce a breaking change?

• No. The project now follows the licensing recommendations much closer to the letter.

Other information:

[analytophile]

looks good. Sorry for the late review! Only need to change version number or remove it so this doesn't have to be regularly updated.

[analytophile]

So I don't break anything with my novice Git status- do you want to update the branch to merge with the main, and I'll merge the pull request afterwards?

[Zeitsperre]

@analytophile Yes, that's exactly how I'll go forward.

Once a Pull Request is "approved", anyone with write access here can merge a Pull Request, so I'll make the remaining changes and merge when it's ready.

Thanks again!