Add README and improve folder structure

tests/difference/core/quint_model/README.md

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+This folder contains a Quint model for the core logic of Cross-Chain Validation (CCV).
+
+### File structure
+The files are as follows:
+- ccv.qnt: Contains the type definitions, data structures, functional logic for CCV.
+The core of the protocol.
+- ccv_statemachine.qnt: Contains the state machine layer for CCV. Very roughly speaking, this could be seen as "e2e tests".
+Also contains invariants.
+- ccv_test.qnt: Contains unit tests for the functional layer of CCV.
+- libararies/*: Libraries that don't belong to CCV, but are used by it.
+
+### Model details
+
+To see the data structures used in the model, see the ProtocolState type in ccv.qnt.
+
+The "public" endpoints of the model are those functions that take as an input the protocol state, and return a Result.
+Other functions are for utility.
+
+The parameters of the protocol are defined as consts in ccv.qnt.
+