Docs: add a short 'How it works' section (cash-flow replay + DRIP + XIRR)

[DanMat]

Goal

People on Hacker News / Reddit ask how the numbers are computed. A short "How it works" section pre-empts that and builds trust.

What to do

Add a concise section to the README (or docs/HOW_IT_WORKS.md linked from the README) explaining, in plain language:

• The event-driven replay: contributions and ex-dividend dates are merged into one chronological stream (see src/divvy/engine.py).
• DRIP: dividends are reinvested into the same holding at that day's close.
• Money-weighted XIRR vs. simple total return, and why it matters for lumpy contributions.
• The contribution-free NAV used for risk metrics (drawdown/volatility) — see bucket_nav() in report.py.

Keep it short (a few short paragraphs or a bullet list). No code changes required — great non-code first contribution.

Acceptance criteria

• A clear, accurate "How it works" section renders in the README/docs.