[#170] checksum: Add examples

Signed-off-by: Pavel Karpy <carpawell@nspcc.ru>

checksum/example_test.go

@@ -0,0 +1,34 @@
+package checksum
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"fmt"
+	"math/rand"
+
+	"github.com/nspcc-dev/neofs-api-go/v2/refs"
+)
+
+func ExampleCalculate() {
+	payload := []byte{0, 1, 2, 3, 4, 5, 6}
+	var cs Checksum
+
+	Calculate(&cs, SHA256, payload)
+	Calculate(&cs, TZ, payload)
+}
+
+func ExampleChecksum_WriteToV2() {
+	var (
+		csRaw [sha256.Size]byte
+		csV2  refs.Checksum
+		cs    Checksum
+	)
+
+	rand.Read(csRaw[:])
+	cs.SetSHA256(csRaw)
+
+	cs.WriteToV2(&csV2)
+
+	fmt.Println(bytes.Equal(cs.Value(), csV2.GetSum()))
+	// Output: true
+}