docs: add README with project overview and usage examples

README.md

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+# irtt-rs
+
+`irtt-rs` is a Rust implementation of an IRTT-compatible client.
+
+It is not the upstream IRTT project. For the original implementation, protocol background, and broader project documentation, see:
+
+<https://github.com/heistp/irtt>
+
+## Status
+
+This repository currently provides:
+
+- `irtt-cli`, a command-line IRTT-compatible client
+- `irtt-client`, a Rust library for running client sessions and consuming events
+- finite and continuous probe runs
+- human, simple, machine-readable, and RTT-only output modes
+- optional local summary statistics
+
+Server support is not implemented.
+
+## Install
+
+Requires Rust 1.75 or newer.
+
+From a local checkout:
+
+```sh
+git clone https://github.com/Lochnair/irtt-rs.git
+cd irtt-rs
+cargo install --path crates/irtt-cli
+```
+
+Or run it directly without installing:
+
+```sh
+cargo run -p irtt-cli -- <server>
+```
+
+## CLI usage
+
+Basic test:
+
+```sh
+irtt-cli <server>
+```
+
+Set duration and interval:
+
+```sh
+irtt-cli <server> --duration 30s --interval 100ms
+```
+
+Run continuously:
+
+```sh
+irtt-cli <server> --duration 0
+```
+
+Use a specific output mode:
+
+```sh
+irtt-cli <server> --output human
+irtt-cli <server> --output simple
+irtt-cli <server> --output machine
+irtt-cli <server> --output rtt-us
+```
+
+For available options:
+
+```sh
+irtt-cli --help
+```
+
+## Machine output
+
+Use `--output machine` for line-oriented `key=value` output intended for scripts, monitoring, and autorate consumers.
+
+Example:
+
+```sh
+irtt-cli <server> --duration 0 --interval 250ms --output machine
+```
+
+Each line represents one client event and starts with an `event` field, for example:
+
+```text
+event=echo_reply seq=4 logical_seq=4 remote=203.0.113.10:2112 raw_rtt_us=12400 effective_rtt_us=12100 adjusted_rtt_us=12100 server_processing_us=300 dscp=0 ecn=0 kernel_rx_ns=none
+```
+
+Consumers should match on `event=...` and read the fields they need. Unknown fields should be ignored.
+
+For consumers that only need RTT values, use:
+
+```sh
+irtt-cli <server> --output rtt-us
+```
+
+## Library usage
+
+`irtt-client` can be used directly from Rust code.
+
+Example `Cargo.toml` dependency from a local checkout:
+
+```toml
+[dependencies]
+irtt-client = { path = "crates/irtt-client" }
+```
+
+Minimal managed-client example:
+
+```rust
+use std::time::Duration;
+
+use irtt_client::{
+    ClientConfig, ClientEvent, ManagedClient, SubscriberConfig,
+};
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let config = ClientConfig {
+        server_addr: "netperf-eu.bufferbloat.net:2112".to_owned(),
+        duration: Some(Duration::from_secs(10)),
+        interval: Duration::from_secs(1),
+        ..ClientConfig::default()
+    };
+
+    let (session, events) =
+        ManagedClient::start_with_subscription(config, SubscriberConfig::default())?;
+
+    while let Ok(event) = events.recv() {
+        match event {
+            ClientEvent::EchoReply { seq, rtt, .. } => {
+                println!("seq={seq} rtt_us={}", rtt.effective.as_micros());
+            }
+            ClientEvent::SessionClosed { .. } => break,
+            _ => {}
+        }
+    }
+
+    let outcome = session.join()?;
+    println!("session ended: {:?}", outcome.end_reason);
+
+    Ok(())
+}
+```