To cite package 'smoothie' in publications use:

  Gilleland, E. (2013) Two-dimensional kernel smoothing: Using the R
  package smoothie. NCAR Technical Note, TN-502+STR, 17pp.,
  doi:10.5065/D61834G2.

A BibTeX entry for LaTeX users is

  @TechReport{,
    title = {Two-dimensional kernel smoothing: Using the R package smoothie},
    author = {Eric Gilleland},
    institution = {NCAR Technical Note, TN-502+STR, 17pp.},
    year = {2013},
    doi = {10.5065/D61834G2},
  }