fathead_overview.md: limit the languages that we can use

Limit languages to Perl, Python, JavaScript, or Ruby for maintenance reasons. We need to be able to update it if necessary, so we're sticking to languages that the DDG Staff knows for now. When the community grows, then we could expand the language support. But for now, it's necessary that we know the program very well so that we can support and maintain it.
Not your fault, but I just noticed the double `` around output.txt. Will remove..