Preparing for 1st submission

1. usethis::use_release_issue() (adds checklist issue to GH)
2. Polish NEWS.md
3. usethis::use_cran_comments()
4. Update installation instructions in README
5. Proofread DESCRIPTION
6. @return & @examples for all @export
7. cph in Authors@R
8. Check licensing
9. Review extrachecks