pages/data: initial paragraphs
_config.yml: comment out BASE_PATH and ASSET_PATH as not needed.
Fill out pages/data on how to include datasets in an R pkg
pages/tests: add note: write test then fix bug

 - In testing page, should emphasize that whenever you find a bug, you
   should first write a test and _then_ fix the bug.
pages/vignettes: Initial stab at text; needs links
An initial bit of text on writing tests
pages/vignettes: Fill in links
pages/vignettes: fix typos
pages/data: Mention use of .R, .txt, .csv files for data