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

deploytool/cmd/luci_deploy/param.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 (
         "regexp"
         "strings"
 
@@ skipping 31 matching lines @@
                         // this function.
                         return errors.Reason("undefined parameter %(key)q").D("key", key).Err()
                 }
         }
 
         // Finish the string and return.
         parts = append(parts, v[sidx:])
         *vp = strings.Join(parts, "")
         return nil
 }