add README.md

Author: gfx

README.md

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+# sqldef.github.io
+
+Website and WebAssembly build of [sqldef](https://github.com/sqldef/sqldef) - an idempotent schema management tool for MySQL, PostgreSQL, SQLite3, and SQL Server.
+
+## What is this?
+
+This repository contains:
+- A WASM build of sqldef's DDL diff functionality
+- A web demo at https://sqldef.github.io/
+
+## How it works
+
+The Go source (`sqldef-wasm.go`) compiles to WebAssembly and exposes a `diff` function that compares two SQL schemas and generates migration DDLs.
+
+## Development
+
+```sh
+# Build the WASM file
+make sqldef.wasm
+
+# Start local dev server at http://localhost:6543
+make dev
+
+# Update Go dependencies
+make update
+
+# Format and lint
+make format
+make lint
+```
+
+## License
+
+MIT
+
+Copyright (c) sqldef team