Add example code snippet to the README

Create a copy-able snippet to get folks going.

Include a link to the Developer Book.

README.md

@@ -18,6 +18,62 @@ The easiest way to help grow the Veilid network is to run your own node. Every u
 
 To run such a node, establish a Debian or Fedora based VPS and install the veilid-server service. To make this process simple we are hosting package manager repositories for .deb and .rpm packages. See the [installing](./INSTALL.md) guide for more information.
 
+## Building on Veilid
+
+If you want to start using Veilid for your own app, take a look at the [Developer Book](https://veilid.gitlab.io/developer-book/).
+
+A basic example using `veilid-core` and `tokio` might look like this.
+
+```rust
+use std::sync::Arc;
+use veilid_core::VeilidUpdate::{AppMessage, Network};
+use veilid_core::{VeilidConfigBlockStore, VeilidConfigInner, VeilidConfigProtectedStore, VeilidConfigTableStore, VeilidUpdate};
+
+#[tokio::main]
+async fn main() {
+    let update_callback = Arc::new(move |update: VeilidUpdate| {
+        match update {
+            AppMessage(msg) => {
+                println!("Message: {}", String::from_utf8_lossy(msg.message().into()));
+            }
+            Network(msg) => {
+                println!("Network: Peers {:}, bytes/sec [{} up] [{} down]", msg.peers.iter().count(), msg.bps_up, msg.bps_down)
+            }
+            _ => {
+                println!("{:?}", update)
+            }
+        };
+    });
+
+    let config = VeilidConfigInner {
+        program_name: "Example Veilid".into(),
+        namespace: "veilid-example".into(),
+        protected_store: VeilidConfigProtectedStore {
+            // avoid prompting for password, don't do this in production
+            always_use_insecure_storage: true,
+            directory: "./.veilid/block_store".into(),
+            ..Default::default()
+        },
+        block_store: VeilidConfigBlockStore {
+            directory: "./.veilid/block_store".into(),
+            ..Default::default()
+        },
+        table_store: VeilidConfigTableStore {
+            directory: "./.veilid/table_store".into(),
+            ..Default::default()
+        },
+        ..Default::default()
+    };
+
+    let veilid = veilid_core::api_startup_config(update_callback, config).await.unwrap();
+    println!("Node ID: {}", veilid.config().unwrap().get_veilid_state().config.network.routing_table.node_id);
+    veilid.attach().await.unwrap();
+    // Until CTRL+C is pressed, keep running
+    tokio::signal::ctrl_c().await.unwrap();
+    veilid.shutdown().await;
+}
+```
+
 ## Development
 
 If you're inclined to get involved in code and non-code development, please check out the [contributing](./CONTRIBUTING.md) guide. We're striving for this project to be developed in the open and by people for people. Specific areas in which we are looking for help include: