vignette: add documentation / example for parallel computation

[lgeistlinger]

This is very “nice-to-have”: For me it would have been helpful to have an example with parallel execution where you show how to specify the number of workers - since I did not want to use all cores of our cluster. To a first-time user of BiocParallel (or parallel in R generally), it took me a while to understand the BiocParallel syntax.