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

deploytool/cmd/luci_deploy/staging.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 (
         "strings"
 
         "github.com/luci/luci-go/common/errors"
@@ skipping 63 matching lines @@
                                         D("path", d).Err()
                         }
                 }
                 link := d.File(pkgComponents[len(pkgComponents)-1])
                 if err := link.SymlinkFrom(src, true); err != nil {
                         return errors.Annotate(err).Reason("failed to create GOPATH link").Err()
                 }
         }
         return nil
 }