tools/android/loading Improve BigQuery template documentation

tools/android/loading/cloud/frontend/README.md

 # Appengine Frontend for Clovis
 
 [TOC]
 
 ## Usage
 
 Visit the application URL in your browser, and upload a JSON dictionary with the
 following keys:
 
 -   `action` (string): the action to perform. Only `trace` and `report` are
@@ skipping 35 matching lines @@
 
 Finds all the traces in the specified bucket and generates a report in BigQuery.
 
 - `trace_bucket` (string): Name of the storage bucket where trace databases can
   be found. It can be either absolute or relative to the `storage_bucket`
   specified in the backend parameters.
 
 This requires an existing `clovis_dataset.report` BigQuery table that will be
 used as a template. The schema of this template is not updated automatically and
 must match the format of the report (as generated by `report.py`).
-To update the schema manually, delete and recreate the `clovis_dataset.report`
-from the BigQuery web interface as an empty table with the new schema.
+See [how to update the schema manually][7].
 
 ## Development
 
 ### Design overview
 
 This is a [python AppEngine][5] application using [Flask][6].
 
 -   Appengine configuration:
     -   `app.yaml` defines the handlers. There is a static handler for all URLs
     in the `static/` directory, and all other URLs are handled by the
@@ skipping 52 matching lines @@
 # Install dependencies in the lib/ directory.
 pip install -r requirements.txt -t lib
 # Deploy.
 gcloud preview app deploy app.yaml
 ```
 
 ### Updating the Database Schema
 
 When a change is made to the dictionary returned by report.py, the BigQuery
 database schema must be updated accordingly.
-Then, to actually update the schema, run:
+
+To update the schema, run:
 
 ```shell
 bq update \
   --schema \
     $CHROMIUM_SRC/tools/android/loading/cloud/frontend/bigquery_schema.json \
   -t clovis_dataset.report
 ```
 
 Adding a new field is harmless, but don't modify existing ones.
 
+If the above command does not work, this is probably because you are doing more
+than adding fields.
+In this case, you can delete and recreate the `clovis_dataset.report` table from
+the [BigQuery web interface][8]:
+-   Expand `clovis_dataset` from the left menu, and delete the `report` table.
+-   Create a new table in `clovis_dataset`, and call it `report`.
+-   Set `Location` to `None` in order to create an empty table.
+-   Click `Edit as Text` in the `Schema` section , and  paste the contents of
+    `bigquery_schema.json` there.
+
 
 [1]: https://cloud.google.com/sdk
 [2]: https://cloud.google.com/appengine/docs/python/taskqueue
 [3]: https://cloud.google.com/appengine/docs/python/config/queue
 [4]: ../backend/README.md#Deploy-the-code
 [5]: https://cloud.google.com/appengine/docs/python
 [6]: http://flask.pocoo.org
+[7]: #Updating-the-Database-Schema
+[8]: https://bigquery.cloud.google.com