RFR: add README.md

[Andrei Rybak]

Add a README.md file in Markdown format with a link to the project on openjdk.org and with a banner of welcome.jpg file.  README.md is nicely formatted by repository browsers like [GitHub][1] on the landing page of the repository.
    
[1]: https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/about-readmes

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How the file is rendered can be seen on GitHub: https://github.com/openjdk/duke/blob/1d68763c12afac7f5a3b15f02937afe567d0256a/README.md

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Commit messages:
 - add README.md

Changes: https://git.openjdk.org/duke/pull/2/files
 Webrev: https://webrevs.openjdk.org/?repo=duke&pr=2&range=00
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/duke/pull/2.diff
  Fetch: git fetch https://git.openjdk.org/duke.git pull/2/head:pull/2

PR: https://git.openjdk.org/duke/pull/2