Add warning about METEOR version and flag-induced score variance (#677)

This pull request adds a call-out block at the top of `evaluate/metrics/meteor/README.md` to inform users about significant score discrepancies in METEOR evaluations. Variations up to ±10 points can occur due to differences between versions 1.0 and 1.5, as well as the use of specific flags (`-l`, `-norm`, `-vOut`). By highlighting this issue, users are encouraged to specify the Java package version and document the flags used to ensure reproducibility. The information is based on findings from Lübbers (2024), available at [https://github.com/cluebbers/Reproducibility-METEOR-NLP](https://github.com/cluebbers/Reproducibility-METEOR-NLP).