add vignette

[lwarode]

• create vignette and introduce sample workflow with package and connect functions to each other

[lwarode]

• (maybe) introduce analytical possibilities, e.g. with quanteda and/or tidytext
  --> quanteda offers wordscores and wordfish options

• quanteda and tidytext require specific import formats in their functional framework, so the vignette could show how the data objects obtained with the R6 class needs to be manipulated for analyses in both package frameworks