added startup message to package with vignette, citation, and mention to look at duplicates warning man page, #77
added pacage level man file, closes #79
added note about duplicate occurrence records in a new file spocc_duplicates, #77
added note about duplicate records to occ() fxn man page, #77
added note about duplicate records to README.md file, #77
added note about duplicate records to vignette, #77