The Text inside README.md or any other Markdown files added to the project's bundle, simply are not formatted when previewed in XCode.
Almost all of the opensource projects have instructions in Markdown at the top level of the project & even Apple use MD in their sample projects.
Magically, the .md files in Apple sample projects are formatted as expected as you can see below, but this isn't the case in any other project. I've also checked the file's settings in the XCode inspector and it didn't differ from the ones existing in the Apple code samples.
This is how we are used to view Markdowns in XCode.
And this is is the README.md in HappyBeam which is formatted.