add usage example (#3)

example_test.go

@@ -0,0 +1,50 @@
+package netjail_test
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"log"
+	"net/http"
+	"net/netip"
+
+	"github.com/stealthrocket/netjail"
+)
+
+func ExampleTransport() {
+	// This rule set only allows connecting to a private IPv4 network.
+	ctx := netjail.ContextWithRules(context.Background(),
+		&netjail.Rules{
+			Allow: []netip.Prefix{
+				netip.MustParsePrefix("10.0.0.0/8"),
+			},
+		},
+	)
+
+	client := &http.Client{
+		Transport: &netjail.Transport{
+			New: func() *http.Transport {
+				return http.DefaultTransport.(*http.Transport).Clone()
+			},
+		},
+	}
+
+	r, err := http.NewRequestWithContext(ctx, "GET", "http://localhost/", nil)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	if r, err := client.Do(r); err != nil {
+		if !errors.Is(err, netjail.ErrDenied) {
+			log.Fatal(err)
+		} else {
+			fmt.Println("Access Denied")
+		}
+	} else {
+		r.Body.Close()
+		fmt.Println("Access Granted")
+	}
+
+	// Output:
+	// Access Denied
+}