Add references to description description

[jhollway]

If there are references describing the methods in your package, please
add these in the description field of your DESCRIPTION file in the form
authors (year) doi:...
authors (year) arXiv:...
authors (year, ISBN:...)
or if those are not available:https:...
with no space after 'doi:', 'arXiv:', 'https:' and angle brackets for
auto-linking.
(If you want to add a title as well please put it in quotes: "Title")