appview/markup: restrict README candidate matching
Restricts README detection to `README` or `README.<single_extension>` filenames only, preventing files like `README.Music.md` being selected to be rendered over `README.md`
Fixes [#604](https://tangled.org/tangled.org/core/issues/604)
Signed-off-by: Smit Patil <smit@smit.codes>