Adds my name to description file as contributor

[mbarnfield]

Added:

person("Matthew", "Barnfield", role = "ctb",
email = "m.barnfield@qmul.ac.uk"))

Please ensure the following before submitting a PR:

• if suggesting code changes or improvements, open an issue first
• for all but trivial changes (e.g., typo fixes), add your name to DESCRIPTION
• for all but trivial changes (e.g., typo fixes), documentation your change in NEWS.md with a parenthetical reference to the issue number being addressed
• if changing documentation, edit files in /R not /man and run devtools::document() to update documentation
• add code or new test files to /tests for any new functionality or bug fix
• make sure R CMD check runs without error before submitting the PR