Soooo, in the Jekyll version of this theme, it's in the _config.yml, but I'm not 100% sure how Hugo works. Is it the description item in theme.toml?
I don't know enough about Hugo or blogdown to know how to set the home page up, sorry 
EDIT: I'm guessing that using an .Rmd as the home page works the same way as using a .md or other file—maybe you just name it index.Rmd, pop it in the root folder (actually, it could be content/_index.rmd in Hugo), and make sure it starts with a frontmatter block like:
+++
layout = "default"
+++
Here is my awesome home page content! With some R chunks, or whatever I wanna put in there...
The layout would be one of the ones provided by the theme... sorry, I know that's a little vague! If someone with more Hugo knowledge wants to jump in, they might be able to give you a better answer 