Incorporate Levi's sweave2rmd_conversiontemplate into contributions.Rmd

[jwokaty]

Let's make a new document to describe the process of using Levi's https://github.com/lwaldron/sweave2rmd_conversiontemplate. We will need to figure out the process of how to use it then when/where does a user go back to follow the contribute.Rmd.

I think we just need to be careful because it could be confusing for someone who isn't familiar with Github and we still need to keep the manual instructions for vignettes in the case that the conversion template doesn't work.