Document min_os_version

Change-Id: Ie4b2ecd7e64a19d17fb84790b844fc96751621a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2246545
Auto-Submit: Jonathan Backer <backer@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779334}

testing/variations/README.md

@@ -92,13 +92,18 @@ the experiment group name.
 > client code, the `name` **must** exactly match the client code and the server
 > config.
 
-The remaining keys -- `enable_features`, `disable_features`, and `params` -- are
-optional.
+The remaining keys -- `enable_features`, `disable_features`, `min_os_version`,
+and `params` -- are optional.
 
 `enable_features` and `disable_features` indicate which features should be
 enabled and disabled, respectively, through the
 [Feature List API][FeatureListAPI].
 
+`min_os_version` indicates a minimum OS version level (e.g. "10.0.0") to apply
+the experiment. This string is decoded as a `base::Version`. The same version is
+applied to all platforms. If you need different versions for different
+platforms, you will need to use different studies.
+
 `params` is a dictionary mapping parameter name to parameter value.
 
 > Reminder: The variations framework does not actually fetch any field trial