Add gravity flow demo

backend/wordpress/wp-content/plugins/gravityflow-master/tests/acceptance-tests/readme.txt

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+The easiest way to run the acceptance tests is to run them in Docker.
+
+Requirements:
+- docker
+- docker compose
+
+Run the acceptance tests using start.sh. For example:
+bash start.sh
+
+You can watch Chrome perform the tests via VNC on localhost:5900. On OSX open vnc://localhost:5900 in Safari. Password: secret.
+
+Optionally, you can remove the containers when you've finished:
+docker-compose down