chore: update README.md with creds how-to

Author: matiasedgeandnode

README.md

@@ -42,6 +42,19 @@ brew install uv
    # - EXAMPLES_A2A_SELLER__PAY_TO_ADDRESS: Ethereum address to receive payments
    ```
 
+4. **Obtain required credentials:**
+
+   **Google API Key** (required for seller example):
+
+   1. Visit [Google AI Studio](https://aistudio.google.com/app/apikey)
+   2. Sign in with your Google account
+   3. Click "Get API key" or "Create API key"
+   4. Copy the generated key and add it to your `.env` file
+
+   **USDC Testnet Tokens** (required for buyer example):
+
+   Use the [Circle USDC testnet faucet](https://faucet.circle.com) to obtain test USDC for your buyer wallet address.
+
 ### 💡 Optional: direnv
 
 For automatic environment variable loading, install [direnv](https://direnv.net/):