You can use an R markdown file as Readme file. In Domino, the project overview is rendered from the README.md file.
If you are using RStudio to compose your .Rmd file, you can also generate the .md file from it. To do so,
1) Open the .Rmd file in RStudio
2) Click on the Settings gear icon (next to Knit) and click on Output Options
3) Click on the Advanced tab, check Keep markdown source file and click Ok
4) Use 'Knit' to create the .md file which you can sync back to your project files for Domino to render in your project overview.
Alternatively, you can also copy and paste the contents of your README.Rmd file into the README.md file.