Make mask token explicit

Make the mask token explicit to prevent the error `No mask_token (<mask>) found on the input` in the Hosted Inference API. See [related issue](https://github.com/huggingface/transformers/issues/11884#issuecomment-851590598) for more context on this fix.

README.md

@@ -93,6 +93,7 @@ tags:
   - Twitter
   - Multilingual
 license: "apache-2.0"
+mask_token: "[MASK]"
 ---
 
 # TwHIN-BERT: A Socially-Enriched Pre-trained Language Model for Multilingual Tweet Representations