contrib/git-sync: add comment

contrib/git-sync/main.go

+/*
+Copyright 2014 Google Inc. All rights reserved.
+
+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.
+*/
+
+// git-sync is a command that periodically sync a git repository to a local directory.
+
 package main // import "github.com/GoogleCloudPlatform/kubernetes/git-sync"
 
 import (
@@ -67,6 +85,7 @@ type Repo struct {
 	lastRev    string
 }
 
+// NewRepo initalize a local bare repository mirror.
 func NewRepo() (*Repo, error) {
 	mirrorRepoPath := path.Join(*dest, ".git")
 	_, err := os.Stat(mirrorRepoPath)
@@ -92,6 +111,9 @@ func NewRepo() (*Repo, error) {
 	}, nil
 }
 
+// Sync fetch new revision from the origin remote in the bare repository
+//      create a new checkout named after the revision
+//	update HEAD symlink to point to the new revision
 func (r *Repo) Sync() {
 	cmd := exec.Command("git", "fetch", "origin", *branch)
 	cmd.Dir = r.mirrorPath