added a note about performance issues for low-resource languages

README.md

@@ -1093,7 +1093,7 @@ model-index:
 
 ## Model Details
 
-Neural machine translation model for translating from Multiple languages (mul) to unknown (deu+eng+nld).
+Neural machine translation model for translating from Multiple languages (mul) to unknown (deu+eng+nld). Note that many of the listed languages will not be well supported by the model as the training data is very limited for the majority of the languages. Translation performance varies a lot and for a large number of language pairs it will not work at all.
 
 This model is part of the [OPUS-MT project](https://github.com/Helsinki-NLP/Opus-MT), an effort to make neural machine translation models widely available and accessible for many languages in the world. All models are originally trained using the amazing framework of [Marian NMT](https://marian-nmt.github.io/), an efficient NMT implementation written in pure C++. The models have been converted to pyTorch using the transformers library by huggingface. Training data is taken from [OPUS](https://opus.nlpl.eu/) and training pipelines use the procedures of [OPUS-MT-train](https://github.com/Helsinki-NLP/Opus-MT-train).
 **Model Description:**
@@ -1126,6 +1126,8 @@ This model can be used for translation and text-to-text generation.
 
 Significant research has explored bias and fairness issues with language models (see, e.g., [Sheng et al. (2021)](https://aclanthology.org/2021.acl-long.330.pdf) and [Bender et al. (2021)](https://dl.acm.org/doi/pdf/10.1145/3442188.3445922)).
 
+Also note that many of the listed languages will not be well supported by the model as the training data is very limited for the majority of the languages. Translation performance varies a lot and for a large number of language pairs it will not work at all.
+
 ## How to Get Started With the Model
 
 A short example code: