GitLab

* [R-package] parallelize compilation in CMake-based builds

* Apply suggestions from code review
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>

* working on adding -j

* pass -j through to install.libs.R

* add docs on -j

* use -j4

* Update R-package/README.md
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>
Co-authored-by: Nikita Titov <nekit94-08@mail.ru>