Docs: explain OpenAI judge and Serper backend configuration

Author: MagellaX

README.md

@@ -51,7 +51,9 @@ Configuration
   - `model`: `base_model` name (placeholder until you wire a real LLM).
   - `reward`: weights `alpha, beta, gamma, delta` for the combined objective.
   - `search`: `backend` (currently `local`) and `top_k`.
+    - To use web search via Serper: set `backend: serper` and export `SERPER_API_KEY`.
   - `data`: `sample_corpus` path and an example `questions` list.
+  - `judge`: choose `heuristic` (default) or `openai` (requires `OPENAI_API_KEY`).
 
 CLI commands
 - Demo search (offline local index)