To cite package 'rain' in publications use:

  Paul F. Thaben and Pål O. Westermark. Detecting Rhythms in Time
  Series with RAIN.  J Biol Rhythms. 2014 Dec; 29(6): 391–400

A BibTeX entry for LaTeX users is

  @Article{,
    journal = {Journal of Biological Rhythms},
    title = {Detecting Rhythms in Time Series with RAIN},
    author = {Paul F. Thaben and Pål O. Westermark},
    year = {2014},
    volume = {29},
    number = {6},
    pages = {391--400},
    doi = {10.1177/0748730414553029},
  }