AcceptabilityChecker doc (#3124)

parlai/crowdsourcing/utils/README.md

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+# Crowdsourcing utilities
+
+## `acceptability.AcceptabilityChecker`
+
+* Used to make sure that a worker's messages throughout a conversation meet certain criteria (not too short, not all caps, not a lot of repetition, safety, etc.)
+### How to add a new check
+- Add the code for this in `.check_messages()`, inside a `if 'check_name' in violation_types:` condition
+- Add the name of the check to `self.possible_violation_types`; otherwise, this check will not be recognized, and an error will be raised if the check is specified when calling `.check_messages()`!
+- To use the check: add the name of the check to the `violation_types` arg when calling `.check_messages()`