Update MASS.md

Author: Xiaoxue-xx

instructions/Seq2Seq/MASS.md

@@ -0,0 +1,12 @@
+## MASS
+
+You can fine-tune a MASS model from HuggingFace through ``model=MASS``, ``model_path=<hf-or-local-path>``, ``dataset=<dataset-name>``. 
+
+Example usage:
+
+```bash
+python run_textbox.py \
+    --model=MASS \
+    --model_path=RUCAIBox/mass-base-uncased \
+    --dataset=samsum
+```