Added R and Rcpp documentation links
Added R and Rcpp documentation links

NCBI-Hackathons/SRA2R,NCBI-Hackathons/SRA2R,rosachoe/SRA2R,NCBI-Hackathons/SRA2R,rosachoe/SRA2R,rosachoe/SRA2R,NCBI-Hackathons/SRA2R,rosachoe/SRA2R