[R] citing a package?

[Martin Henry H. Stevens]

How do I cite a package (not R itself - I know how to do that)? Any 
thoughts or links?
Many thanks in advance?
Hank Stevens

Dr. Martin Henry H. Stevens, Assistant Professor
338 Pearson Hall
Botany Department
Miami University
Oxford, OH 45056

Office: (513) 529-4206
Lab: (513) 529-4262
FAX: (513) 529-4243
http://www.cas.muohio.edu/botany/bot/henry.html
http://www.muohio.edu/ecology/
http://www.muohio.edu/botany/