doc(target_chains/starknet): add readme

target_chains/starknet/contracts/README.md

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+# Pyth Starknet Contract
+
+This directory contains the Pyth contract on [Starknet](https://www.starknet.io/).
+
+## Installation
+
+1. Install Scarb (the Cairo and Starknet development toolchain) by following [the installation instructions](https://docs.swmansion.com/scarb/download).
+2. Install Starknet Foundry by following [the installation instructions](https://foundry-rs.github.io/starknet-foundry/getting-started/installation.html).
+
+The `.tool-versions` file in this directory specifies the tool versions used by the contract.
+
+## Testing
+
+Run `snforge test` in this directory to run the contract unit tests and integration tests.
+
+## Local deployment
+
+1. Install Starkli (a cli tool for Starknet) by following [the installation instructions](https://github.com/xJonathanLEI/starkli).
+2. Install Katana (a local Starknet node) by following [the installation instructions](https://book.starknet.io/ch02-04-katana.html).
+3. Run the `deploy/local_deploy` setup script.