Testing Varations: Expand on the Field Trial Testing Config

BUG=NONE

Review-Url: https://codereview.chromium.org/2343003002
Cr-Commit-Position: refs/heads/master@{#419184}

testing/variations/README

@@ -5,7 +5,16 @@ This directory contains field trial configurations that are used
 to ensure testing coverage of the experiments defined in the
 fieldtrial_testing_config_*.json files.
 
+The keys in these JSON files should match the Field Trial study name.
+The group names should match the experiment group names.
+
+Chrome features must be explicitly enabled using 'enable_features',
+as the variations framework does not actually fetch the Field Trial
+definitions.
+
 Field trial experiments specified in these configs are set to be
 enabled by default for Chromium builds. This configuration is also
 used for perf bots and browser tests in the waterfall.
 
+Note that these apply specifically for Chromium builds, and Chrome
+branded / official builds do not use these definitions.