Added stub of usage vignette
Added stub of usage vignette

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