deploytool: Add README.md, migrate docs to it.

deploytool/cmd/luci_deploy/manage.go

 // Copyright 2016 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 package main
 
 import (
         "fmt"
         "strings"
 
@@ skipping 215 matching lines @@
         err := a.runWork(c, func(w *work) error {
                 a.dp.params.stage = deployCommit
                 return a.dp.deploy(w)
         })
         if err != nil {
                 logError(c, err, "Failed to update GAE parameters.")
                 return 1
         }
         return rv
 }