Restore testdata/README.md

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Author: mpkorstanje

testdata/README.md

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+# Acceptance test data
+
+The junit xml formatter uses the examples from the [cucumber compatibility kit](https://github.com/cucumber/compatibility-kit)
+for acceptance testing. These examples consist of `.ndjson` files created by
+the [`fake-cucumber` reference implementation](https://github.com/cucumber/fake-cucumber).
+
+* The `.njdon` files are copied in by running `npm install`.
+* The expected `.xml` files are created by running the
+  `MessagesToJunitXmlWriterAcceptanceTest#updateExpectedXmlReportFiles` test.
+
+We ensure the `.ndjson` files stay up to date by running `npm install` in CI
+and verifying nothing changed.
+
+Should there be changes, these tests can be used to update the expected data:
+ * Java: `MessagesToJunitXmlWriterAcceptanceTest.updateExpectedXmlReportFiles`