My logic here is to always delete `README.html` in case the user previously had a `README.md`, but removed it (so we don't have a stale html file). Though it's also the case that the link won't be added in the list template if the `README.md` is missing either.

If `README.md` is missing, the `File.read` failure will get handled by the second `write_readme` which is a no-op. If that's too obtuse, I could handle with a `case` or `cond` directly in `generate_readme`.