Add README documentation

Add a minimal README documentation to explain the usage and project's
motivation.

Signed-off-by: Yutaro Hayakawa <yutaro.hayakawa@isovalent.com>

README.md

@@ -1,2 +1,102 @@
 # go-ra
-Pure Go IPv6 Router Advertisement daemon
+
+[![Go Reference](https://pkg.go.dev/badge/github.com/YutaroHayakawa/go-ra.svg)](https://pkg.go.dev/github.com/YutaroHayakawa/go-ra)
+
+Provides the `ra` package that implements a router-side functionality of IPv6
+Neighbor Discovery mechanism
+([RFC4861](https://datatracker.ietf.org/doc/html/rfc4861) and related RFCs). It
+also provides a stand-alone daemon `gorad` and CLI tool `gora` to interact with
+it. While the existing
+[mdlayher/ndp](https://pkg.go.dev/github.com/mdlayher/ndp) package provides a
+low-level protocol functionalities (packet encoding, raw-socket wrapper, etc),
+`go-ra` implements an unsolicited and solicited advertisement machinery and
+declarative configuration interface on top of it.
+
+## Usage
+
+### As a library
+
+```go
+// Build a configuration
+config := ra.Config{
+	  Interfaces: []*ra.InterfaceConfig{
+		    {
+			      Name: "eth0",
+			      // Send unsolicited RA once a second
+			      RAIntervalMilliseconds: 1000, // 1sec
+		    },	
+	  },
+}
+
+// Create an RA daemon
+daemon, _ := ra.NewDaemon(&config)
+
+// Run it
+ctx, cancel := context.WithCancel(context.Background())
+go daemon.Run(ctx)
+
+// Get a running status
+status := daemon.Status()
+for _, iface := range status.Interfaces {
+    fmt.Printf("%s: %s (%s)\n", iface.Name, iface.State, iface.Message)
+}
+
+// Change configuration and reload
+config.Interfaces[0].RAIntervalMilliseconds = 2000 // 2sec
+err := daemon.Reload(ctx, &config)
+if err != nil {
+		panic(err)
+}
+
+// Stop it
+cancel()
+```
+
+### As a stand-alone daemon
+
+Create a configuration file.
+
+```yaml
+interfaces:
+- name: eth0
+  raIntervalMilliseconds: 1000 # 1sec
+```
+
+Start daemon. You need a root privilege to run it.
+
+```bash
+
+$ sudo gorad -f config.yaml
+```
+
+Get status.
+
+```bash
+$ gora status
+Name         Age    TxUnsolicited    TxSolicited    State      Message
+eth0         22s    21               1              Running
+```
+
+Modify and reload configuration
+
+```bash
+$ gora reload -f config.yaml
+```
+
+## Use Case
+
+If you’re looking for the full-featured RA daemon, maybe
+[radvd](https://radvd.litech.org/) is a better option at this point. The
+advantages of `go-ra` is it is written in pure go and easy to integrate with
+your existing go project.
+
+Our original motivation was use it with
+[gobgp](https://pkg.go.dev/github.com/osrg/gobgp/v3) library to do [BGP
+Unnumbered](https://github.com/osrg/gobgp/blob/master/docs/sources/unnumbered-bgp.md)
+(see our [integration test](integration_tests/gobgp_unnumbered_test.go)) which
+for us, makes sense to reinvent the RA daemon to not introduce an external
+non-go dependency. That’s why currently it only implements very limited
+functionalities enough for our immediate use case.
+
+However, we’re eager to expand the use cases if there’s any demand. The project
+is designed with the extensibility in mind. Contribution is always welcome!