Add citation example

[mroberge]

There should be an example of how to cite hydrocloud in README.md, the ?Help page, and perhaps in some of the code pages too. Maybe the wiki too?

Roberge, M. C., McGuire, M. P., & Lian, J. (2017). HydroCloud: A Web Application for Exploring Stream Gage Data. Journal of Open Research Software, 5(1), 18. DOI: http://doi.org/10.5334/jors.173

• README.md
• ?help page
• inside of the code, maybe?
• on one of the wiki pages?