docs: FAQ entry on using caveman with ponytail (#599)

Most-asked question, and the answer was only ever in the skill file
(skills/ponytail/SKILL.md: "pair with Caveman for terse prose"), never
in the README. Added to both READMEs as the first FAQ entry.

Co-authored-by: Dietrich Gebert <dgebert@Dietrichs-MacBook-Pro.local>