Add CITATION.cff file

[Jaewon Chung]

Create and add a CITATION.cff file for the JMLR paper.

See About CITATION files.

[Hashenko]

Hi, I saw that this issue is still open and improved the previous author's file a bit. In particular, I added more information about the authors, licence, version, abstract and keywords.

[M.liaichi]

hi, does this issue solved ?