portable file names per R Check
'name' param documented
doc param mismatch for `f` vs `prismfile`
function return documentation.
document `f` param and function return
build fail fix. must suggest or depend
doc for `month` and `prismfile`
clarification on `name` boolean param
added `library(prism)` as install had eval=F
doc updates
adding `ggplot2` package to readme.rmd
Merge pull request #21 from jread-usgs/master

fixes for R check and documentation