ci: add build README.md to document.yaml

[dshemetov]

No description provided.

[dsweber2]

Seems like something we'll only see the actual results of once it's merged, unfortunately, but seems sane

[dshemetov]

Something (I'm going to guess devtools::build_readme() or some related function) installed a Git Hook into my repo that complained if I did not commit both the md and rmd.

➜  epidatr git:(ds/readme) ✗ git add README.md 
➜  epidatr git:(ds/readme) ✗ git commit -m "as"
README.Rmd and README.md should be both staged
use 'git commit --no-verify' to override this check

So this CI is extra, but I figure it can't hurt. Also this CI will actually push a commit to the branch (before the PR merge), so it's a little more handy.