Handle case when a directory does not have a README.md file in it.

[younata]

Currently, when a directory does not have a README.md file in it, there is no subsection generated for the other items in that directory. This causes those items to be added as subitems to the lexicographically-previous item.

For example, the following directory structure:

some_file.md
subsection/
    foo.md
    bar.md

Would generate the following SUMMARY.md:

[Some File](some_file.md)
 - [Foo](foo.md)
 - [Bar](bar.md)

Which is confusing to read.

Easiest way to handle this is to log and throw an error if we detect a directory structure that is non-empty, but doesn't contain a README.md file.