Create contributing guidelines

[kelly-sovacool]

Changes

Create contributing guidelines for collaborators

Issues

Resolves #23

PR Checklist

(~Strikethrough~ any points that are not applicable.)

• [x] This comment contains a description of changes with justifications, with any relevant issues linked.
• ~[ ] Write unit tests for any new features, bug fixes, or other code changes.~
• ~[ ] Update the docs if there are any API changes (roxygen comments, vignettes, readme, etc.).~
• ~[ ] Update NEWS.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/~
• [x] Run devtools::check() locally and fix all notes, warnings, and errors.

[kelly-sovacool]

Will add specific git commands and screenshots from GitHub per @phoman14's feedback.

[kelly-sovacool]

@phoman14 the updated guide is now ready for your review: https://github.com/CCBR/reneeTools/blob/contrib-docs/.github/CONTRIBUTING.md