To make Xcode render markdown, create an empty .plist file named .xcodesamplecode.plist in your .xcodeproj/ folder, or .workspace/ folder if you have one.
- With the file selected, go to Editor > Syntax Coloring and select None
- Select another file
- Select the markdown file again
- With the file selected, go to Editor > Syntax Coloring and select Default For Filetype (should be Markdown)
- Select another file
- Select the markdown file again
Bit of a nuisance. Unfortunately I couldn't find any key bindings for Editor > Syntax Coloring either.