Add shim skeleton code

This allows runtime authors to quickly bootstrap new shim
implementations.

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>

runtime/v2/example/cmd/main.go

@@ -0,0 +1,29 @@
+// +build linux
+
+/*
+   Copyright The containerd 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 main
+
+import (
+	"github.com/containerd/containerd/runtime/v2/example"
+	"github.com/containerd/containerd/runtime/v2/shim"
+)
+
+func main() {
+	// init and execute the shim
+	shim.Run("io.containerd.example.v1", example.New)
+}