First release to attach gtex via {piggyback}

This release facilitates the upload of the GTEX archive retrieved using the {yarn} R package. As the archive is 1.4 GB it cannot be uploaded as a file, hence we are utilising the heuristic of attaching it as a release associated archive.

More information about this approach can be found here Karthik Ram's Rstudio conference 2019 talk (9:50) and in the github repo of {piggyback} https://github.com/ropensci/piggyback.