docs(readme): note default embedding model in source-install section

Source-build contributors lost this implicit info when the long-form env-var
table moved into docs/CONFIGURATION.md. One-line pointer keeps them oriented
without re-inlining the full reference.

Addresses review feedback on #208.

Author: HumanBean17

README.md

@@ -175,6 +175,8 @@ python3 -m venv .venv
 
 The `cocoindex` package is **only** needed for lifecycle commands that run the indexer (`init`, `increment`, `reprocess`, `erase`). Search and MCP navigation work without it.
 
+The default embedding model is `sentence-transformers/all-MiniLM-L6-v2` (downloaded on first `init`). Override via the `EMBEDDING_MODEL` env var — see [`docs/CONFIGURATION.md` §1](./docs/CONFIGURATION.md#1-environment-variables).
+
 ---
 
 ## Roadmap (graph layer)