Add example code, update README.md

A quick example is very useful for people.

Author: cgwalters

README.md

@@ -1,3 +1,11 @@
 # Go library for parsing Fedora CoreOS streams
 
-See https://github.com/coreos/fedora-coreos-tracker/issues/98#issuecomment-683150940
+See the [Fedora CoreOS documentation](https://docs.fedoraproject.org/en-US/fedora-coreos/getting-started/)
+for basic information about streams.
+
+This is a Go library which exposes API to decode streams into Go structs,
+as well as a convenience API to find the URL for a given stream.
+
+# Example usage
+
+See [example code here](example/example.go).

example/example.go

@@ -0,0 +1,61 @@
+// Package main contains an example use of this library; it
+// prints the current Fedora CoreOS EC2(AWS) x86_64 AMI in the
+// us-east-2 region.
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"io/ioutil"
+	"net/http"
+	"os"
+
+	"github.com/coreos/stream-metadata-go/fedoracoreos"
+	"github.com/coreos/stream-metadata-go/stream"
+)
+
+const (
+	targetArch = "x86_64"
+	region     = "us-east-2"
+)
+
+func run() error {
+	streamurl := fedoracoreos.GetStreamURL(fedoracoreos.StreamStable)
+	resp, err := http.Get(streamurl.String())
+	if err != nil {
+		return err
+	}
+	body, err := ioutil.ReadAll(resp.Body)
+	resp.Body.Close()
+	if err != nil {
+		return err
+	}
+
+	var fcosstable stream.Stream
+	err = json.Unmarshal(body, &fcosstable)
+	if err != nil {
+		return err
+	}
+	arch, ok := fcosstable.Architectures[targetArch]
+	if !ok {
+		return fmt.Errorf("No %s architecture in stream", targetArch)
+	}
+	awsimages := arch.Images.Aws
+	if awsimages == nil {
+		return fmt.Errorf("No %s AWS images in stream", targetArch)
+	}
+	var regionVal stream.AwsRegionImage
+	if regionVal, ok = awsimages.Regions[region]; !ok {
+		return fmt.Errorf("No %s AWS images in region %s", targetArch, region)
+	}
+	fmt.Printf("%s\n", regionVal.Image)
+
+	return nil
+}
+
+func main() {
+	if err := run(); err != nil {
+		fmt.Fprintf(os.Stderr, "%v\n", err)
+		os.Exit(1)
+	}
+}