[FLINK-9594][E2E tests] Improve docs for new test runner

This closes #6172.

flink-end-to-end-tests/README.md

@@ -28,11 +28,45 @@ where <flink dir> is a Flink distribution directory.
 You can also run tests individually via
 
 ```
-$ FLINK_DIR=<flink dir> flink-end-to-end-tests/test-scripts/test_batch_wordcount.sh
+$ FLINK_DIR=<flink dir> flink-end-to-end-tests/run-single-test.sh your_test.sh arg1 arg2
 ```
 
 ## Writing Tests
 
-Have a look at test_batch_wordcount.sh for a very basic test and
-test_streaming_kafka010.sh for a more involved example. Whenever possible, try
-to put new functionality in common.sh so that it can be reused by other tests.
+### Examples
+Have a look at `test_batch_wordcount.sh` for a very basic test and
+`test_streaming_kafka010.sh` for a more involved example. Whenever possible, try
+to put new functionality in `common.sh` so that it can be reused by other tests.
+
+### Adding a test case
+In order to add a new test case you need add it to either `test-scripts/run-nightly-tests.sh` and / or `test-scripts/run-pre-commit-tests.sh`. Templates on how to add tests can be found in those respective files.
+
+_Note: If you want to parameterize your tests please do so by adding multiple test cases with parameters as arguments to the nightly / pre-commit test suites. This allows the test runner to do a cleanup in between each individual test and also to fail those tests individually._
+
+### Passing your test
+A test is considered to have passed if it:
+- has exit code 0
+- there are no non-empty .out files (nothing was written to stdout / stderr by your Flink program)
+- there are no exceptions in the log files
+- there are no errors in the log files
+
+_Note: There is a whitelist for exceptions and errors that do not lead to failure, which can be found in the `check_logs_for_errors` and `check_logs_for_exceptions` in `test-scripts/common.sh`._
+
+Please note that a previously supported pattern where you could assign a value the global variable `PASS` to have your tests fail **is not supported anymore**.
+
+### Cleanup
+The test runner performs a cleanup after each test case, which includes:
+- Stopping the cluster
+- Killing all task and job managers
+- Reverting config to default (if changed before)
+- Cleaning up log and temp directories
+
+In some cases your test is required to do to some *additional* cleanup, for example shutting down external systems like Kafka or Elasticsearch. In this case it is a common pattern to trap a `test_cleanup` function to `EXIT` like this:
+
+```sh
+function test_cleanup {
+    # do your custom cleanup here
+}
+
+trap test_cleanup EXIT
+```