[DOC] Create a vignette showing how to generate the image of the README

[tbouffard]

The image shows an example of a BPMN diagrams explaining a functional scenario (Email Voting) and involves a lot of overlays (both on shapes and edges). It will be also easier to know how we generated the image: currently, we have to remember how to generate it.

The R script used to generate the image is described in #45 and #72. It must be fixed for #84
Vignette documentation: https://r-pkgs.org/vignettes.html

Resources about vignettes

examples from the ggfun package

• https://github.com/YuLab-SMU/ggfun/blob/master/vignettes/ggfun.Rmd
• https://cran.r-project.org/web/packages/ggfun/vignettes/ggfun.html
• https://cran.r-project.org/web/packages/ggfun/index.html