Add project README with description, features, and links

README.md

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-# SCD-Bond-Calculator
+# SCD Bond Calculator
 
+A self-contained bond return calculator widget for [Stirchley Community Development (SCD)](https://www.stirchley.coop/) — a community housing co-operative in Stirchley, Birmingham, UK.
+
+Potential investors can model returns on SCD community bonds across four interest models (compound/simple × at-maturity/paid-yearly), see a year-by-year breakdown, and understand the security tier and UK tax implications of their investment.
+
+## Features
+
+- **Three inputs** — investment amount (£20k–£10M), interest rate (0–3.5%), term (5–30 years)
+- **Four payout methods** — compound at maturity, compound paid yearly, simple at maturity, simple paid yearly
+- **Summary cards** — total repayment, total interest, per-year income, effective annual rate (CAGR)
+- **Security tier** — unsecured (< £150k), tertiary charge (£150k–£500k), secondary charge (> £500k)
+- **Tax estimates** — basic / higher / additional rate UK income tax (before Personal Savings Allowance)
+- **Year-by-year table** — full breakdown with collapsible detail and totals row
+
+## Usage
+
+Open `calc.html` in any browser. No build step or server required — the widget is a single, self-contained HTML file that can be embedded directly as a WordPress Custom HTML block.
+
+## Bond offer
+
+Refer to the [SCD Bond Offer Document](https://www.stirchley.coop/wp-content/uploads/2026/05/SCD-Bond-Offer-2026-25-May-2026.pdf) for full terms, risks, and legal details.
+
+## License
+
+AGPL-3.0 — see [LICENSE](LICENSE).