add .env instructions

Author: xzrderek

eval_protocol/proxy/.env.example

@@ -0,0 +1,2 @@
+# In order to set other model providers keys for proxy, make a copy, rename to .env, and fill here
+OPENAI_API_KEY=sk-proj-xxx

eval_protocol/proxy/docker-compose.yml

@@ -19,6 +19,9 @@ services:
     platform: linux/amd64
     container_name: litellm-backend
     command: ["--config", "/app/config.yaml", "--port", "4000", "--host", "0.0.0.0"]
+    # If you want to be able to use other model providers like OpenAI, Anthropic, etc., you need to set keys in .env file.
+    env_file:
+      - .env  # Load API keys from .env file
     environment:
       - LANGFUSE_PUBLIC_KEY=dummy  # Set dummy public and private key so Langfuse instance initializes in LiteLLM, then real keys get sent in proxy
       - LANGFUSE_SECRET_KEY=dummy