Track current documentation in git

[dshemetov]

Addresses #15. Might want to do a documentation pass before merging, but I'm unsure how to preview the docs from a PR. We need to set GH pages to host the docs, maybe?

[brookslogan]

Previewing docs can be done by

devtools::build_manual()

and looking at the pdf, or

devtools::install()
help(package="delphi.epidata") # for index of help topics --- this isn't supported when we are using devtools::load_all
[plus cleanup:]
remove.packages("delphi.epidata")
[restart R session]
devtools::load_all(export_all=FALSE) # for help on individual topics, we can use install or load_all
## browse through index)
help(delphi.epidata::covid_hosp_facility)
help(delphi.epidata::covid_hosp_facility_lookup)
[...]

[brookslogan]

Looks good to me. The documentation looks pretty bare-bones and might need a few corrections or completions, but I think it makes sense to go ahead and merge anyway.

For previewing documentation / documentation website easily within a PR, maybe we can check with covidcast developers who might already know how. (Also for how they publish the documentation website for main somewhere, if that's not already happening.) One way to preview the pdf manual would be to track the manual in the repo as well, but that's a bit inconvenient/forgetfullness-prone as devtools::document does not build the pdf manual by default and it takes a bit of time.