Initial commit: async Rust client for the Invoice Ninja v5 API

Covers Clients, Invoices, Quotes, Credits, Payments, Products, Expenses,
Projects, Tasks, and Vendors, each with list, list_all, get, create,
update, and delete, generated by the resource_methods! macro.

Every model carries a #[serde(flatten)] extra map so fields the API
returns but the crate does not model survive a get/update round-trip
instead of being silently dropped.

Includes a Nix dev shell (Rust via rust-overlay), a GitHub Actions CI
workflow running build/test/fmt/clippy, and round-trip tests for the
extra-preservation invariant.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -0,0 +1,44 @@
+# invoiceninjars
+
+An async Rust client for the [Invoice Ninja](https://www.invoiceninja.com/) v5 API.
+
+## Development
+
+Enter the dev shell (Rust toolchain via `rust-overlay`):
+
+```sh
+nix develop
+```
+
+## Usage
+
+```rust
+use invoiceninjars::{InvoiceNinjaClient, models::ListParams};
+
+let client = InvoiceNinjaClient::new(
+    "https://invoicing.co/api/v1", // or your self-hosted base URL
+    std::env::var("IN_API_TOKEN")?,
+)?;
+
+let clients = client.list_clients(&ListParams::new().per_page(50)).await?;
+```
+
+List endpoints are paginated. `list_*` returns a single page; `list_all_*`
+follows pagination and returns every matching record:
+
+```rust
+let all_clients = client.list_all_clients(&ListParams::new()).await?;
+```
+
+Run the example (requires `IN_API_TOKEN`, optionally `IN_BASE_URL`):
+
+```sh
+IN_API_TOKEN=... cargo run --example list_clients
+```
+
+## Coverage
+
+Clients, Invoices, Quotes, Credits, Payments, Products, Expenses, Projects,
+Tasks, and Vendors, each with `list`, `list_all`, `get`, `create`, `update`,
+and `delete`. Add more resources by adding a model under `src/models/` and a
+`resource_methods!` invocation in `src/client.rs`.