Add example plugin for third-party plugin use

plugin/example/main.go

@@ -0,0 +1,36 @@
+// This is an example plugin. In packer 0.9.0 and up, core plugins are compiled
+// into the main binary so these files are no longer necessary for the packer
+// project.
+//
+// However, it is still possible to create a third-party plugin for packer that
+// is distributed independently from the packer distribution. These continue to
+// work in the same way. They will be loaded from the same directory as packer
+// by looking for packer-[builder|provisioner|post-processor]-plugin-name. For
+// example:
+//
+//    packer-builder-docker
+//
+// Look at command/plugin.go to see how the core plugins are loaded now, but the
+// format below was used for packer <= 0.8.6 and is forward-compatible.
+package main
+
+import (
+	"github.com/mitchellh/packer/builder/amazon/chroot"
+	"github.com/mitchellh/packer/packer/plugin"
+	"github.com/mitchellh/packer/post-processor/docker-push"
+	"github.com/mitchellh/packer/provisioner/powershell"
+)
+
+func main() {
+	server, err := plugin.Server()
+	if err != nil {
+		panic(err)
+	}
+	// Choose the appropriate type of plugin. You should only use one of these
+	// at a time, which means you will have a separate plugin for each builder,
+	// provisioner, or post-processor.
+	server.RegisterBuilder(new(chroot.Builder))
+	server.RegisterPostProcessor(new(dockerpush.PostProcessor))
+	server.RegisterProvisioner(new(powershell.Provisioner))
+	server.Serve()
+}

plugin/example/main_test.go

@@ -0,0 +1,3 @@
+// Core Packer plugins included a _test.go file to make sure they compiled
+// correctly when running packer tests. For custom plugins this file is optional.
+package main