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

deploytool/api/deploy/checkout.proto

 // 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.
 
 syntax = "proto3";
 
 package deploy;
 
 /**
  * Source layout configuration file.
  *
  * Each Source checkout may include a textproto layout file named
- * "luci-deploytool.cfg". If present, this file will be loaded and used to
+ * "luci-deploy.cfg". If present, this file will be loaded and used to
  * integrate the source into the deployment.
  *
  * Uncooperative repositories, or those not owned by the author, may have the
  * same effect by specifying the SourceLayout in the Source's layout definition.
  */
 message SourceLayout {
   /**
    * Init is a single initialization operation execution.
    */
   message Init {
@@ skipping 51 matching lines @@
    *
    * For example, given checkout:
    *   path: gosrc/my/thing
    *   go_package: github.com/example/mything
    *
    * This will add a GOPATH entry:
    * src/github.com/example/mything => <root>/gosrc/my/thing
    */
   string go_package = 2;
 }