This function creates a .gitignore file at the root of the project based on
a template (specific to R). If a .gitignore is already present, files to
be untracked by git are just added to this file.
See also
Other development functions:
add_dependencies(),
add_github_actions_check(),
add_github_actions_citation(),
add_github_actions_codecov(),
add_github_actions_codemeta(),
add_github_actions_document(),
add_github_actions_pkgdown(),
add_github_actions_render(),
add_r_depend(),
add_to_buildignore()
