Update README.md

README.md

@@ -210,9 +210,9 @@ In order to implement new scenarios:
 1. Create a new file as a new Python scenario file in the `scenarios` folder.
 2. Within the scenario file, create a `Scenario` class, e.g. `YourScenario`.
 3. `YourScenario` should implement `get_instances`, a method returning a 
-   list of `Instance` objects which each have a list of (potentially only one)
-   `Reference` answers. You may have `CORRECT_TAG` in a `Reference` instance's
-   tags argument, indicating that it is the correct answer. In addition, you 
+   list of `Instance` objects. Each `Instance` must have a list of (potentially one)
+   `Reference` answers: a correct answer may be indicated with a `CORRECT_TAG` in 
+   a `Reference` instance's `tags` argument. In addition, you 
    must specify the `split` of the `Instance` as one of `TRAIN_SPLIT`,
    `VALID_SPLIT`, or `TEST_SPLIT` constants as in `scenario.py`.
 4. Note that you need not enumerate every possible correct answer (nor must