README.md: Add DOI and citation guide

[li-boxuan]

Closes #4760

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Thank you for contributing to JanusGraph!

In order to streamline the review of the contribution we ask you
to ensure the following steps have been taken:

For all changes:

• Is there an issue associated with this PR? Is it referenced in the commit message?
• Does your PR body contain #xyz where xyz is the issue number you are trying to resolve?
• Has your PR been rebased against the latest commit within the target branch (typically master)?
• Is your initial contribution a single, squashed commit?

For code changes:

• Have you written and/or updated unit tests to verify your changes?
• If adding new dependencies to the code, are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0?
• If applicable, have you updated the LICENSE.txt file, including the main LICENSE.txt file in the root of this repository?
• If applicable, have you updated the NOTICE.txt file, including the main NOTICE.txt file found in the root of this repository?

For documentation related changes:

• Have you ensured that format looks appropriate for the output in which it is rendered?

[FlorianHockmann]

While looking how other software projects handle this, I learned that GitHub apparently has built in support for this with CITATION.cff files: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-citation-files

So we could also move the information (apart from the badge) into such a file probably instead of adding it to the README as it should only affect a small percentage of our users.
But that's just an idea I'm also completely fine with having it in the README.

[li-boxuan]

CITATION.cff seems nice but it's also unfortunate that the "Cite this repository" is very easy to miss. I didn't even know the existence of this feature after citing a handful of GitHub repositories...

TensorFlow also has a dedicated page for citation instruction https://www.tensorflow.org/about/bib which I guess serves this purpose well enough, but most citeable reposities I have seen usually have citation section in README.