Support README.Rmd
### Problem to solve
RMarkdown is a format that extend Markdown to support literate programming with R and other languages. Would be great if GitLab supported README.Rmd like it does for README.md.
### Target audience
- Delaney, Development Team Lead, https://design.gitlab.com/research/personas#persona-delaney
- Sasha, Software Developer, https://design.gitlab.com/research/personas#persona-sasha
### Further details
RMarkdown could be processed just like Markdown.
### Proposal
Add `.Rmd` extension to the list of supported file extensions.
### What does success look like, and how can we measure that?
How many repositories have `README.md`
### Links / references
issue