Improvement for installation document

I've found a quick way to get started on Ubuntu is:
```sh
sudo apt install r-base r-cran-testthat
```
Is that something to add to the installation document? Too OS-specific?

Find the repository's installation document and review how operating-system-specific setup instructions are handled. Evaluate the Ubuntu command using apt for installing r-base and r-cran-testthat; done means the document clearly includes the agreed instruction or leaves the existing guidance unchanged with the rationale recorded.

Written by the indexing model from the issue text.