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

deploytool/cmd/luci_deploy/build.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 86 matching lines @@
 
                 if err := python.exec(scriptPath, args...).cwd(buildDir).check(w); err != nil {
                         return errors.Annotate(err).Reason("failed to execute [%(scriptPath)s]").D("scriptPath", scriptPath).Err()
                 }
                 return nil
 
         default:
                 return errors.Reason("unknown build type %(type)T").D("type", t).Err()
         }
 }