Add README.md with example

README.md

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+# macos-routing-table
+
+A basic parser and route testing API for macOS devices.  Parses a static or live routing table (via the `netstat` command) and provides an interface to inspect tht table, and to determine which gateway and interface would be used to route traffic to a given address.
+
+Analyzes both the IPv4 and IPv6 routing tables. Minimal support for zones (parsing only at this time).
+
+## Example - find the gateway for an address
+
+``` rust
+use anyhow::Result;
+use macos_routing_table::{RouteEntry, RoutingTable};
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    let rt = RoutingTable::load_from_netstat().await?;
+    let addr = "1.1.1.1".parse()?;
+
+    if let Some(RouteEntry {
+        net_if, gateway, ..
+    }) = rt.find_route_entry(addr)
+    {
+        println!("{addr:?} => {gateway} via {net_if}");
+    } else {
+        println!("No route to {addr:?}");
+    }
+
+    Ok(())
+}
+```