Update README.md

Add comment of max_seq_len to sentence transformers, especially for efficient training:
```python
# Optionally, lower the maximum sequence length for lower memory usage
# model.max_seq_length = 8192
```

README.md

@@ -77,6 +77,9 @@ model = SentenceTransformer("Qwen/Qwen3-Embedding-8B")
 #     tokenizer_kwargs={"padding_side": "left"},
 # )
 
+# Optionally, lower the maximum sequence length for lower memory usage
+# model.max_seq_length = 8192
+
 # The queries and documents to embed
 queries = [
     "What is the capital of China?",
@@ -282,4 +285,4 @@ If you find our work helpful, feel free to give us a cite.
   journal={arXiv preprint arXiv:2506.05176},
   year={2025}
 }
-```
+```