README has no run steps or CLI example

[Param-Harrison]

Where

README.md

What's missing or wrong

The README describes what splitbill is but never says how to actually run
it: no install step, no bun run dev command, no port number, and no
example of using the CLI (splitbill groups, splitbill balances <group>,
splitbill export <group>). A new contributor has to read package.json
and src/cli.ts to figure out how to start the app.

Expected

A "Run it" section with:

• bun install
• bun run dev and the URL it serves (http://localhost:3200)
• One CLI example, e.g. bun run src/cli.ts groups

Done when

The README has these steps and they work when followed on a clean clone.