Specify rietveld as code review tool in contributors guide.

CONTRIBUTING.md

 <!-- Copyright 2015 The Chromium Authors. All rights reserved.
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file.
 -->
 # Code of Conduct
 
 We follow the [Chromium code of conduct](
 https://chromium.googlesource.com/chromium/src/+/master/CODE_OF_CONDUCT.md) in
 our our repos and organizations, mailing lists, and other communications.
 
@@ skipping 10 matching lines @@
 
 ```
 cd catapult
 git checkout -t -b foo origin/master
 ... edit files ...
 git commit -a -m "New files"
 ```
 
 Once you're ready for a review do:
 
-`git cl upload`
+`git cl upload --rietveld`
 
 Once uploaded you can view the CL in Rietveld and **request a review** by
 clicking the 'publish & mail' link, and adding a reviewer from the
 [OWNERS](/OWNERS) file. You can also click the "CQ Dry Run" link to run all
 the tests on your change.
 
 If you get review feedback, edit and commit locally and then do another upload
 with the new files. Before you commit you'll want to sync to the tip-of-tree.
 You can either merge or rebase, it's up to you.
 
@@ skipping 70 matching lines @@
 
 Chromium's DEPS file needs to be rolled to the catapult revision containing your
 change in order for it to appear in Chrome's about:tracing or other
 third_party/catapult files. Follow the [directions for rolling DEPS](/docs/rolling-deps.md)
 to do this.
 
 # Adding a new project
 
 Please read the [directory structure guide](/docs/directory-structure.md)
 to learn the conventions for new directories.