Enhanced citation support

We've added enhanced support for CITATION.cff files to GitHub. CITATION.cff files are plain text files with human- and machine-readable citation information, and with this new feature, GitHub parses this information into convenient formats such as APA and BibTeX that can be copied by others.

Under the hood, we’re using the ruby-cff RubyGem to parse the contents of the CITATION.cff file and build a citation string that is then shown in the GitHub user interface. Special thanks to the gem creators @sdruskat @jspaaks and @hainesr who worked with us to build this.