Start tracking Rd files on GitHub

[jgabry]

rstanarm doesn't track Rd files on GitHub, which results in the doc not being included in various situations (see e.g. @ermeel86's comment #442 (comment), but there are also other cases) . @bgoodri I forget why this decision was made initially, but rstanarm might be the only R package on GitHub that doesn't track the Rd files (at least it's hard to find another one). I don't think there's a good reason not to track them at this point, so I think we should start doing it. Objections?