Updates functional test readme (#2492) (#2545)

Updates the Readme to call out the change in functional testing
for OpenSearch Dashboards

Issue resolved:
#2462

Signed-off-by: Ashwin P Chandran <ashwinpc@amazon.com>
(cherry picked from commit a672ee1)

Co-authored-by: Ashwin P Chandran <ashwinpc@amazon.com>

CHANGELOG.md

@@ -82,6 +82,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 * Remove extra typo from README. ([#2403](https://github.com/opensearch-project/OpenSearch-Dashboards/pull/2403))
 * Add sample config for multi data source feature in yml template. ([#2428](https://github.com/opensearch-project/OpenSearch-Dashboards/pull/2428))
 * README.md for dataSource and dataSourceManagement Plugin ([#2448](https://github.com/opensearch-project/OpenSearch-Dashboards/pull/2448))
+* Updates functionl testing information in Testing.md ([#2492](https://github.com/opensearch-project/OpenSearch-Dashboards/pull/2492))
 
 ### 🛠 Maintenance
 

TESTING.md

@@ -43,6 +43,9 @@ To run specific integration tests, pass the path to the test:
 `yarn test:jest_integration [test path]`
 
 ### Functional tests
+
+Functional testing in OpenSearch Dashboards is migrating to the [opensearch-dashboards-functional-test](https://github.com/opensearch-project/opensearch-dashboards-functional-test) repository. All new functional tests should be written there. When modifying a file that affects an existing functional test, the old test should be migrated to the new repository. The rest of this section outlines how to run the existing functional tests in the repository.
+
 To run all functional tests:
 `yarn test:ftr`
 To run specific functional tests, you can run by CI group: