Add two basic examples of usage, as go tests.

examples_test.go

+package ipld_test
+
+import (
+	"bytes"
+	"fmt"
+	"os"
+
+	"github.com/ipld/go-ipld-prime/codec/dagjson"
+	"github.com/ipld/go-ipld-prime/node/basic"
+)
+
+func ExampleCreateDataAndMarshal() {
+
+	ns := basicnode.Style.Any // Pick a style for the in-memory data.
+	nb := ns.NewBuilder()     // Create a builder.
+	ma, _ := nb.BeginMap(2)   // Begin assembling a map.
+	ma.AssembleKey().AssignString("hey")
+	ma.AssembleValue().AssignString("it works!")
+	ma.AssembleKey().AssignString("yes")
+	ma.AssembleValue().AssignBool(true)
+	ma.Finish()     // Call 'Finish' on the map assembly to let it know no more data is coming.
+	n := nb.Build() // Call 'Build' to get the resulting Node.  (It's immutable!)
+
+	dagjson.Encoder(n, os.Stdout)
+
+	// Output:
+	// {
+	//	"hey": "it works!",
+	//	"yes": true
+	// }
+}
+
+func ExampleUnmarshalData() {
+	serial := bytes.NewBufferString(`{"hey":"it works!","yes": true}`)
+
+	ns := basicnode.Style.Any   // Pick a stle for the in-memory data.
+	nb := ns.NewBuilder()       // Create a builder.
+	dagjson.Decoder(nb, serial) // Hand the builder to decoding -- decoding will fill it in!
+	n := nb.Build()             // Call 'Build' to get the resulting Node.  (It's immutable!)
+
+	fmt.Printf("the data decoded was a %s kind\n", n.ReprKind())
+	fmt.Printf("the length of the node is %d\n", n.Length())
+
+	// Output:
+	// the data decoded was a Map kind
+	// the length of the node is 2
+}