add GCS support to docs server

chrome/common/extensions/docs/server2/README

 --------
 Overview
 
 This is a Google App Engine server which serves the documentation for Chrome
 apps and extensions. At time of this writing, the primary URL is:
 http://developer.chrome.com/.
 
 
 ---------------------
 Developing the Server
 
 You shouldn't need app engine locally to develop the server, preview.py should
 be sufficient. If for some reason you want to test against the app engine SDK:
 
   1. Download the python Google App Engine SDK from:
      https://developers.google.com/appengine/downloads
 
   2. Run './start_dev_server.py <path/to/dev_appserver.py>'
      (dev_appserver.py is part of the App Engine)
 
   3. View docs at http://localhost:8080/(apps|extensions)/<doc_name>
 
 
+--------------------------------------------
+Using Google Cloud Storage content providers
+
+With preview.py:
+
+  1. create a directory "[...]/server2/local_debug/gcs/<bucketname>" for every
+     gcs bucket referenced in content_providers.json
+
+  2. copy files to the respective local bucket directories. Preview.py has
+     no access to the real Google Cloud Storage.
+
+With start_dev_server.py:
+
+    1. Install gsutils from https://developers.google.com/storage/docs/gsutil
+
+    2. Set gsutil accordingly to the official instructions.
+
+    3. Make sure you have permission to the GCS buckets specified in
+       content_providers.json by running "gsutil ls gs://bucketname"
+
+    4. Get an oauth token (see instructions at the comment of
+       gcs_file_system_provider.py) and save it to the file
+       "[...]/server2/local_debug/gcs_debug.conf"
+
+Remember that the step 4 needs to be repeated every 10 minutes or so,
+because the oauth access token expires quickly.
+
 --------------------
 Deploying the Server
 
 You will need to have access to the http://chrome-apps-doc.appspot.com app.
 Contact aa@chromium.org, erikkay@chromium.org, miket@chromium.org,
 kalman@chromium.org, or ernestd@chromium.org to obtain access.
 
 Once you have access:
 
 1. Increment the version in app.yaml so we can roll back if the update breaks.
 
 2. Run build_server.py. This copies some depenencies from /third_party into the
    server directory so that they get uploaded to App Engine.
 
 3. Run appcfg.py (supplied with the App Engine SDK) to upload the server code:
 
     appcfg.py update .
 
 4. When prompted for your credentials, enter the information for the account
    that has access to the production app.
 
 5. Go to http://www.appspot.com, select the docs project, click "versions" in
    the sidebar, and make the version you just deployed the "default" version.
 
    If you get an error about too many versions when deploying, go into this
    view and delete the version which was deployed the longest time ago.  Then
    try to deploy again.