docs: Add mask filling example

README.md

@@ -18,6 +18,11 @@ When you use the reference API, you must wrap the sentence with `[BOS]` and `[EO
 [BOS] 안녕하세요? 반가워요~~ [EOS]
 ```
 
+You can also test mask filling performance using `[MASK]` token like this.
+```
+[BOS] [MASK] 먹었어? [EOS]
+```
+
 ## Used Datasets
 
 ### [모두의 말뭉치](https://corpus.korean.go.kr/)