chore(.Rbuildignore): remove unnecessary files and directories from t…

…he build process

The following files and directories were removed from the build process:
- .Rproj
- .Rproj.user
- README.Rmd
- LICENSE.md
- CODE_OF_CONDUCT.md
- appveyor.yml
- .travis.yml
- codemeta.json
- doc
- Meta
- codecov.yml
- .github
- _pkgdown.yml
- docs
- pkgdown
- cran-comments.md
- CRAN-RELEASE

These files and directories are not needed for the build process and can be safely ignored.