Document ExPlat experiment setup steps

Author: rachelmcr

docs/EXPERIMENTS.md

@@ -1,6 +1,6 @@
 # Experiments
 
-This framework allows us to develop and test experimental features with different build configurations in release and debug builds. The framework also supports more advanced experimentation with AB testing, using ExPlat in the Tracks library.
+This framework allows us to develop and test experimental features with different build configurations in release and debug builds. The framework also supports more advanced experimentation with AB testing, using the Experimentation Platform (ExPlat) in the Tracks library.
 
 ## Dependencies
 
@@ -11,6 +11,7 @@ This framework allows us to develop and test experimental features with differen
 - Enum `FeatureFlag`: where we define feature flag cases for all the experimental features
 - Protocol `FeatureFlagService` and default implementation `DefaultFeatureFlagService`. The protocol allows mocking feature flag states in unit tests, and the default implementation is based on build configurations
 - Enum `BuildConfiguration`: the current build configuration `BuildConfiguration.current` is currently used in logging
+- Enum `ABTest`: where we define experiment cases for A/B testing
 
 ## Build configurations
 
@@ -21,3 +22,17 @@ The project has three build configurations to match the WooCommerce app: `Debug`
 - `Release-Alpha` build configuration: `ALPHA` value is set. Used for one-off installable builds for internal testing, which we can trigger from a commit in a pull request
 
 In the default implementation of `FeatureFlagService`, some of the feature flags are based on build configurations - enabled in `Debug` and `Release-Alpha` configurations, and disabled in `Release` builds.
+
+## Run an A/B test
+
+To add an ExPlat experiment to the app, add a new case to the `ABTest` enum in the `Experiments` framework.
+
+Once the experiment is added to the app, define the behavior for each variation:
+
+```
+if ABTest.experimentName.variation == .control {
+    // Control logic
+} else {
+    // Treatment logic
+}
+```