Add minimum documentation

[jacquietran]

Nearly there! Last things to do to get to a minimal viable product:

• Check docs for each data set and revise where necessary (I've made some recent changes to the data sets, so there may be variables to remove or add in the corresponding docs)
• Update docs to specify variable definitions that are consistent with the FIBA Statistician's Manual, where relevant: https://www.fiba.basketball/documents/2015/FIBA_Stats_Manual20120920.pdf (H/T to NobleQueemoShabazzElBey on Twitch for the suggestion!)
• Update DESCRIPTION file
• Add package-level documentation, as described in https://rstudio4edu.github.io/rstudio4edu-book/data-pkg.html
• Update README for repo - see issue #16

[jacquietran]

FIBA Statistician's Manual is useful but a bit intense! May update docs in reference to some specific variables that may not be immediately obvious from the variable name alone, e.g., fast-break points.

[jacquietran]

Last things to do in order to close this issue and progress towards alpha release:

• Update DESCRIPTION file
• Add package-level documentation, as described in https://rstudio4edu.github.io/rstudio4edu-book/data-pkg.html