Document the use of comments in input files

[rmzelle]

Ruby line comments (with "#" at the start of the line) work in some files, but I'm not exactly sure which since the generation script doesn't explicitly recognize them as such (e.g. lines starting with "#" _skip.txt
will just cause an identifier mismatch, since style titles or file names never start with a hash).

Would be better to make this clearer in the code and document the behavior.