Create README.md

pcidss/e2e-tests/README.md

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+## Standalone integration tests
+
+This crate contains standalone integration test that simulates an example of transfer that was triggered on-chain. For this
+to work, we need the whole [infrastracture](https://github.com/subclone/payment-processor?tab=readme-ov-file#run-the-demo) for ISO-8583 message processing up and running.
+
+Then, to run the tests:
+
+```sh
+cargo test -p oracle-e2e-tests
+```