Testing Varations: Expand on the Field Trial Testing Config

testing/variations/README

Index: testing/variations/README
diff --git a/testing/variations/README b/testing/variations/README
index 34e2bf500efdbe33338cabeb830b1a8e38a15c8d..8700e1581e08615cbb31b58a6f225a5946d9c18d 100644
--- a/testing/variations/README
+++ b/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.