Update vignette: character-based or unordered pedigrees

[bryo-han]

This is thanks to Ivan P's tests.

We need to point out that pedigreeTools::pedigree expects ordered integer data as input.

In the long run, functionality might be added to pedigreeTools.
Short term, we should mention this requirement in the vignette (and possible add some helper code).

[gregorgorjanc]

@bryo-han note that pedigreeTools::pedigree can work with characters because its leveraging factors - see Rpedigree/pedigreeTools#5