added certora readme

certora/README.md

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+# Certora
+
+Assuming you have already [installed](https://docs.certora.com/en/latest/docs/user-guide/getting-started/install.html) the Prover, to run it against the `priceOracleCounter.spec` file to see the vulnerability highlighted in the [Certora report](https://github.com/aave/aave-v3-core/blob/master/certora/Aave_V3_Formal_Verification_Report_Jan2022.pdf) as 
+    - H2: Confusion of Asset and EMode price feed for liquidations  
+use the following command from the aave-v3-core directory: `certoraRun certoraRun certora/confs/priceOracleCounter.conf`