Guidance on installing older version of GGIR

[vincentvanhees]

A tutorial on how to install older versions of GGIR may be helpful.

We do not provide guidance on this in https://cran.r-project.org/web/packages/GGIR/vignettes/GGIR.html#21_Install_R_and_RStudio

For the non-experienced R user this may feel as if there is no way back once GGIR has been updated.

Further, may need to provide some guidance on how also to install older versions of GGIR dependencies.

To do:

• [x] Explain as part of explanation of config file and the narrative about reproducing analysis.
• [x] Add to getting started brief summary of how to install older versions of GGIR and R