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

deploytool/cmd/luci_deploy/deploy_container_engine.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"
         "sort"
         "strconv"
@@ skipping 657 matching lines @@
                 }
                 switch has, err = kubectl.hasContext(w); {
                 case err != nil:
                         return "", errors.Annotate(err).Reason("failed to confirm Kubernetes context").Err()
                 case !has:
                         return "", errors.Reason("context %(context)q missing after fetching credentials").D("context", kubeCtx).Err()
                 }
         }
         return kubeCtx, nil
 }