feat(testing): add readme for testing

testing/README.md

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+# Substreams Testing
+
+We know testing your Substreams modules can be a pain. So, our goal here is to provide a quick and easy way to run end-to-end tests.
+
+Our script will build the `.spkg` for your Substreams module, index the specified block range, and check if the expected state has been correctly indexed in PostgreSQL.
+
+## How to Test
+
+Here's what you need to do:
+
+1. Get the latest version of our indexer.
+2. Define your tests and expected state.
+3. Run the testing script.
+
+### Get the Latest Version of Tycho (Indexer)
+
+We don't have a direct download link yet. Please reach out to us to get the latest version. Once you have it, place it in the `/testing/` folder.
+
+### Define Your Tests
+
+Define all your tests in a `yaml` file. You can find a template in `substreams/ethereum-template/test_assets.yaml`.
+
+You'll need to:
+
+- Point to the target substreams config file.
+- Specify the expected protocol types.
+- Define your tests.
+
+For each test, the script will index all blocks between `start-block` and `stop-block` and check that the indexed state matches the expected state.
+
+### Run the Script
+
+Once everything is set up, run our script using the CLI.
+
+First, you need a local postgres database
+
+```bash
+docker-compose up -d db
+```
+
+Then export an environment variable for RPC connection
+
+```bash
+export RPC_URL=your-chain-rpc
+```
+
+And finally run the testing script
+
+```bash
+python testing/cli.py --test_yaml_path "./substreams/your-substreams-folder/test_assets.yaml"
+```
+
+You can get the available CLI flags using `--help`