Make vignettes

[PRijnbeek]

Could you convert the two examples into vignettes (pdf) using Rmarkdown.

Add a PackageMaintenance as used here:

https://github.com/OHDSI/PatientLevelPrediction/blob/master/extras/PackageMaintenance.R

[PRijnbeek]

maybe also as R file that can be run as you do for the other advanced documentation

[ioannousolon]

A PackageMaintenance has been added. Vignettes can be found in /doc.