Reduce friction around using roxygen2 + markdown

[batpigandme]

i.e. have use_roxygen_md() insert Roxygen: list(markdown = TRUE) at the bottom of DESCRIPTION without having to run roxygen2md::roxygen2md()

[jennybc]

Or was the suggestion to (also) just put this in our default DESCRIPTION? @hadley

[hadley]

Yeah we should turn it on by default and polish the other function a bit