add runnable example and exclude root package

Signed-off-by: Billy Zha <jinzha1@microsoft.com>

.github/.codecov.yml

@@ -15,4 +15,7 @@ coverage:
   status:
     project:
       default:
-        target: 60%
+        target: 60%
+
+ignore:
+  - "cmd/oras/root"

cmd/oras/root/example_test.go

@@ -0,0 +1,36 @@
+/*
+Copyright The ORAS Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+http://www.apache.org/licenses/LICENSE-2.0
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package root_test
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+// Example gives playable snippets of using oras CLI in command-line scripting
+// via Golang.
+func Example() {
+	// change below array to play with your own cmd args:
+	args := []string{"repo", "ls", "mcr.microsoft.com"}
+	cmd := cmd.NewRoot()
+	cmd.SetArgs(args)
+	fmt.Printf("Executing 'oras %s':", strings.Join(args, " "))
+	err := cmd.Execute()
+	if err != nil {
+		fmt.Errorf("Failed to execute : %w", err)
+		os.Exit(-1)
+	}
+	os.Exit(0)
+}