Add QuantRank500 compose example

[RusUsf]

Adds examples/quantrank500/ plus one README row.

What it is: QuantRank500 is a public record of stock predictions. A
prediction is locked with a cryptographic commitment before the market opens,
revealed at 9:30 AM ET, and settled automatically against exchange trade
records — so nothing is self-reported and nothing can be edited afterwards.
Self-hosting it gives you a private league with the same rules.

Upstream: https://github.com/RusUsf/quantrank500 · live instance:
https://quantrank500.com

Three services: Postgres, the API, and a settlement worker. The API and
worker run the same image and differ only in command.

Two things worth flagging before you review:

1. No image is published to a registry yet, so api and worker use
   build: https://github.com/RusUsf/quantrank500.git. That keeps the compose
   file self-contained — no clone needed — at the cost of a build on first
   start. I saw a couple of existing examples build rather than pull, so I
   hope this is acceptable; happy to switch to a published image once one
   exists if you would rather wait for that.

2. Upstream also ships an nginx service for the static frontend, but it
   bind-mounts files from a checkout, so it cannot work standalone. I left it
   out and said so in the example README rather than ship something that
   half-works.

Section: I put it under Money, Budgeting & Management, next to TRSync,
as the nearest fit. Move it anywhere you prefer.

Validated locally with docker compose config --quiet (the check your CI
runs) — passes.

[l4rm4nd]

Too young project. No stars.

Won't add for now.