document how to ignore system packages with rstudio

rstudio1.md

@@ -201,6 +201,33 @@ that we can install the package via the system.
 
 - https://github.com/eddelbuettel/rcppapt (not yet implemented)
 
+### ignore system packages
+
+If you do not want to use the system packages for some reason you need to
+change the path where R is looking for packages. This must be done at session
+startup and is done by defining environment variables. For rstudio to use the
+environment variables specify them in a file called `.Renviron`. This file can
+either be placed in your home directory or in your working directory. If it is
+in a working directory and rstudio did not start with that working directory,
+change to it and restart R.
+
+Example:
+
+1. Check your current pathes with `.libPaths()`
+1. create directory `~/testsession`
+1. create file `.Renviron` in that directory with the content
+   ```
+   R_LIBS_SITE=/usr/local/lib/R/site-library/
+   ```
+1. login to https://rstudio.idiv.de
+1. `Session -> Set Working Directory -> Choose Directory` and select the
+   `~/testsession` directory
+1. `Session -> Restart R`
+1. Ensure `.libPaths()` contains only the pathes you expect
+1. Now install the packages you require.
+
+[R Startup Documentation](https://stat.ethz.ch/R-manual/R-devel/library/base/html/Startup.html)
+
 
 ## Other servers with R