Explain the value of the test set labels (#5)

- Explain the value of the test set labels (ba51b30b6f03a4b0c3246f9217c2162dfee68a4e)

README.md

@@ -112,7 +112,7 @@ The text in the dataset is in English (`en`).
 
 - `idx`: Monotonically increasing index ID.
 - `sentence`: Complete sentence expressing an opinion about a film.
-- `label`: Sentiment of the opinion, either "negative" (0) or positive (1).
+- `label`: Sentiment of the opinion, either "negative" (0) or positive (1). The test set labels are hidden (-1).
 
 ### Data Splits