Please cite the multcomp package by the following reference:

  Hothorn T, Bretz F, Westfall P (2008). “Simultaneous Inference in
  General Parametric Models.” _Biometrical Journal_, *50*(3), 346-363.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {Simultaneous Inference in General Parametric Models},
    author = {Torsten Hothorn and Frank Bretz and Peter Westfall},
    journal = {Biometrical Journal},
    year = {2008},
    volume = {50},
    number = {3},
    pages = {346--363},
  }