I generally put markdown in /docs. I don't uppercase filenames. Instead I make a documentation generator consume the files so I get a decent navigation in HTML/PDF builds.

We did put non-code into /src for a very long time: It was heredocs, multiline docs, etc. Actually my preference is to put texts close to code and only fallback to /docs/something.md at a conceptual level. Which is probably what the author proposes, given that he sees markdown as primary interface to code.