Update TESTING.md 'Building with extra plugins' documentation

Signed-off-by: Finn Carroll <[email protected]>

TESTING.md

@@ -552,11 +552,17 @@ Apart from using Gradle, it is also possible to gain insight in code coverage us
 
 Please read your IDE documentation for how to attach a debugger to a JVM process.
 
-# Building with extra plugins
+# Testing with plugins
 
-Additional plugins may be built alongside OpenSearch, where their dependency on OpenSearch will be substituted with the local OpenSearch build. To add your plugin, create a directory called `opensearch-extra` as a sibling of OpenSearch. Checkout your plugin underneath `opensearch-extra` and the build will automatically pick it up. You can verify the plugin is included as part of the build by checking the projects of the build.
+To test a plugin with a custom build of OpenSearch, build OpenSearch and use the `customDistributionUrl` setting supported by each plugin to override the OpenSearch distribution.
 
-    ./gradlew projects
+For example, in your OpenSearch repository assemble a custom distribution.
+
+    ./gradlew :distribution:archives:linux-tar:assemble
+
+Then in your plugin repository, substitute in your OpenSearch build
+
+    ./gradlew run -PcustomDistributionUrl="<YOUR-OPENSEARCH-REPO>/OpenSearch/distribution/archives/linux-tar/build/distributions/opensearch-min-3.0.0-SNAPSHOT-linux-x64.tar.gz"
 
 # Environment misc