To cite nlstools in publications use:

  Florent Baty, Christian Ritz, Sandrine Charles, Martin Brutsche,
  Jean-Pierre Flandrois, Marie-Laure Delignette-Muller (2015). A
  Toolbox for Nonlinear Regression in R: The Package nlstools. Journal
  of Statistical Software, 66(5), 1-21. doi 10.18637/jss.v066.i05

A BibTeX entry for LaTeX users is

  @Article{,
    title = {A Toolbox for Nonlinear Regression in {R}: The Package {nlstools}},
    author = {{Florent Baty} and {Christian Ritz} and {Sandrine Charles} and {Martin Brutsche} and {Jean-Pierre Flandrois} and {Marie-Laure Delignette-Muller}},
    journal = {Journal of Statistical Software},
    year = {2015},
    volume = {66},
    number = {5},
    pages = {1--21},
    doi = {10.18637/jss.v066.i05},
  }

Please cite both the package and R when using them for data analysis.
See also ‘citation()’ for citing R.