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

deploytool/cmd/luci_deploy/work.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 (
         "bufio"
         "bytes"
         "fmt"
@@ skipping 278 matching lines @@
 
 func (w *teeWriter) Write(d []byte) (amt int, err error) {
         amt, err = w.base.Write(d)
         if amt > 0 {
                 if _, ierr := w.tee.Write(d[:amt]); ierr != nil {
                         panic(errors.Annotate(ierr).Reason("failed to write to tee writer").Err())
                 }
         }
         return
 }