Updates functional test readme

[ashwin-pc]

Signed-off-by: Ashwin P Chandran [email protected]

Description

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

Issues Resolved

• #2462

Check List

• [ ] All tests pass
  • [ ] yarn test:jest
  • [ ] yarn test:jest_integration
  • [ ] yarn test:ftr
• [ ] New functionality includes testing.
• [x] New functionality has been documented.
• [x] Update CHANGELOG.md
• [x] Commits are signed per the DCO using --signoff