update R version for Mac OS and Windows

.gitlab-ci.yml

+# For Mac OS and Windows use tags 'R' (version 3.6.3) or 'R4' depending of the R version needed.
+# Authors : Jean-François Rey <jean-francois.rey@inrae.fr>
+
 stages: 
   - create
   - tests

README.md

@@ -81,7 +81,9 @@ Four Runners are availables for R Packaging in the shared runners :
 * __R win10 binaries__ : a Windows 10 Virtual Machine with the current R and Rtools version.  
 * __R Mac__ : a Mac OS High Sierra with the current R and Rtools version.  
 
-Check _tags_ in [.gilab-ci.yml](.gitlab-ci.yml) file to enable the runner needed.  
+Check _tags_ in [.gilab-ci.yml](.gitlab-ci.yml) file to enable the runner needed. 
+_R_ tags is for R version 3.6.2 and _R4_ tags is for R verison >= 4.0.0 .  
+> With R version 4.0.0 the Windows toolschain have change and it's no more needed to download binaries of dependendcies as systems libraries.  
 
 This runners come with pre-installer R packages, R tools and system-libraries (bin and dev).  
 See [here](https://gitlab.paca.inra.fr/r-ecosystem/r-docker-images/blob/master/r-base-biosp/Dockerfile) for more informations.