fathead_overview: use a list to show file format
fathead_overview.md: make one list
fathead_overview.md: fix malformed list markdown
fathead_overview.md: add intro section
fathead_overview.md: use a list for dir structure
fathead_overview.md: make Perl example more realistic
fathead_overview.md: Expand intro paragraph
Merge pull request #84 from elebow/fathead-file-format

Expand fathead documentation