Bug fix & documentation updates on CRAN

Changes

• Updated documentation to use Markdown thanks to roxygen2 6.0.0

• Added more links for learning SPARQL in context of Wikidata
  (see help("WDQS", package = "WikidataQueryServiceR"))

  The full list of links is:

  • A beginner-friendly course for SPARQL
  • Building a SPARQL query: Museums on Instagram
  • SPARQL Query Examples for WDQS
  • Using SPARQL to access Linked Open Data by Matthew Lincoln
  • Interesting or illustrative SPARQL queries for Wikidata
  • Wikidata 2016 SPARQL Workshop
  • Wikidata SPARQL Query video tutorial by Navino Evans
  • Learning SPARQL by Bob DuCharme
  • WDQS User Manual

Bug fixes

• Fixed a bug with JSON-formatted results (#3)