Update Contributing vignette

Walk through the contributing vignette:

https://rconnect.chs.usgs.gov/WRTDSplus/articles/Contributing.html from: https://code.usgs.gov/water/wrtdsplus/-/blob/main/vignettes/Contributing.Rmd

and see if anything needs to be updated. There's a section on setting up personal access tokens for example. Since it's now public, we could delete that part.

Feel free to offer suggestions on better ways to collaborate. For instance, we could describe how to create a "fork" instead of working on branches of the main repository. There's a minor hassle doing that with the CI jobs, but it's not too bad. If you want to go that route, let me know.

There might also be spots that could use some editing, I've cobbled this together but haven't done a lot of proof reading, so feel free to make changes.