README should mention that ExecutionMode.CONCURRENT is not supported

[benhubert]

I think, it is very important to mention in the README, that this library does not work, when using @Execution(ExecutionMode.CONCURRENT).

If you run your tests in parallel, a test annotated with @RepeatedIfExceptionsTest(repeats = 10) is only executed one single time but just marked as skipped. This is very dangerous, because the only job of a test is to fail in certain cases. But with this setup, the test will never cause your build to fail, even if the test would have failed all 10 times.