Package needs a vignette

[nturaga]

Hi @arorarshi, as a good first issue, we'll get started on the vignette.

Refer to the following articles:

http://contributions.bioconductor.org/docs.html

I usually write my vignettes in Rmd format and use BiocStyle. This has worked well, because the markdown formatting is easier to work with, and RStudio helps to easily "knit" the document to see a preview.