fix: updating tla+ readme (#443)

consensus/spec/README.md

@@ -1,3 +1,23 @@
-The specification of Pactus consensus algorithm.
-More info can be found here:
-https://pactus.org/learn/consensus/consensus-mechanism.html
+# Consensus specification
+
+This folder contains the consensus specification for the Pactus blockchain,
+which is based on the TLA+ formal language.
+The specification defines the consensus algorithm used by the blockchain.
+
+More info can be found [here](https://pactus.org/learn/consensus/specification/)
+
+## Model checking
+
+To run the model checker, you will need to download and install the [TLA+ Toolbox](https://lamport.azurewebsites.net/tla/toolbox.html),
+which includes the TLC model checker. Follow the steps below to run the TLC model checker:
+
+- Add the `Pactus_Liveness.tla` spec to your TLA+ Toolbox project.
+- Create a new model and specify the temporal formula as `LiveSpec`.
+- Specify the invariants formula as `TypeOK`.
+- Specify the properties formula as `Success`.
+- Since we are checking for liveness, add the `Constraint` formula as the "State Constraint".
+- Define the required constants:
+    - `NumFaulty`: the number of faulty nodes (e.g. 1)
+    - `MaxHeight`: the maximum height of the system (e.g. 2)
+    - `MaxRound`: the maximum round of the consensus algorithm (e.g. 2)
+- Run the TLC checker to check the correctness of the specification.