structure of vignettes / new vignettes

[sa-lee]

We have some good material now for actually using plyranges thanks to the BiocWorkshops which should be linked in both the README and also the intro vignette.

In my opinion, the intro vignette should not reference how things are done / comparisons to GenomicRanges API. That could be split out into a small vignette for users that are actually interested in those details.

Since the overlaps ideas seem to be a point of pain for all users, I think more concrete examples are needed here in a separate vignette. I think we could use the pasillaBamSubset to incorporate the more specialised BAM reading functionality along with this.

So structure would be:

1. introduction (include link to workshops) (this I think of as plyranges for users of tidyverse)
2. plyranges for users of GenomicRanges
3. overlaps

We are introducing two new (short) vignettes, plus a more concrete introduction