Add on README.md instructions to install the correct R environment

[remydecoupes]

I'd like to suggest to add details on how users can install the right R environment (the list of R versions compliant).
Maybe, instructions could be given to use a docker image from the Rocker Project. This project builds image with Rstudio server with different versions of R.

Here is an example with R 4.1.3, rocker/rstudio:4.1.3:

sudo docker run -e ROOT=true -e PASSWORD=OpenGeoHub --rm -p 8787:8787 rocker/rstudio:4.1.3

When the docker is stared, user can connect to RStudio with a web browser: http://localhost:8787, with credentials as follows:

• login: rstudio
• password: OpenGeoHub

And then, they can install system libraries and R packages