Adding a Citation.CFF File

[RichardLitt]

Citation.CFF files are used to encourage citation of software packages.

GitHub now encourages and supports their usage, adding a feature in the UI to "Cite this software".

No relevant issue has been created.

Changes proposed in this pull request:

• Adding a Citation.CFF file at root.

Before submitting

I have not followed all steps in "Making a Pull Request", because this is a doc-only fix. I am sure this will not break the build. No tests are needed for this. It helps you use GitHub's UI better, and I'm not using an AI to write this.

• I've read and followed all steps in the Making a pull request
  section of the CONTRIBUTING docs.
• I've updated or added any relevant docstrings following the syntax described in the
  Writing docstrings section of the CONTRIBUTING docs.
• If this PR fixes a bug, I've added a test that will fail without my fix.
• If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.