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

deploytool/cmd/luci_deploy/path.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 (
         "io/ioutil"
         "os"
         "path/filepath"
@@ skipping 46 matching lines @@
 func withTempDir(f func(string) error) error {
         // Create a temporary directory.
         tdir, err := ioutil.TempDir("", "luci_deploytool")
         if err != nil {
                 return errors.Annotate(err).Reason("failed to create tempdir").Err()
         }
         defer os.RemoveAll(tdir)
 
         return f(tdir)
 }