that’s odd. I just tried to copy a readme over and it worked like a charm (on Ghost v4.17.0). I didn’t view the raw data on Github but went into edit mode and copy/pasted the file’s content
I imagine you could also use a Markdown card in your editor. This one will be cumbersome to edit though.
If you want to migrate a lot of files automatically, you could as well use the Ghost Admin API Documentation - /posts endpoint. Download your Markdown files, compile them to HTML (e.g with Pandoc) and send as many POST requests as necessary