Markdown not your thing? GitHub supports more than just markdown for your Readme markup language. You can view the full list of supported markup languages.
For example, the following readme is written in Creole instead of markdown.
Now if you’re like me and love to break software by trying different combinations of things, you might ask, “What happens if I add both a README.md and a README.creole or other markup language file?” It looks like the first readme file that got created wins (i.e. the readme that is displayed). If you delete the readme file being shown (e.g. README.creole as shown in my example) and you’ve already added a README.md file, the README.md file’s contents will be displayed.