chore: Add document about enable pgvecto.rs extension when database initialization

Signed-off-by: xieydd <xieydd@gmail.com>

README.md

@@ -14,3 +14,16 @@ Container images for [cloudnative-pg](https://cloudnative-pg.io/) with the [pgve
 >     shared_preload_libraries:
 >       - "vectors.so"
 >   ```
+
+> [!IMPORTANT]
+> The `pgvecto.rs` extension is not enabled by default. You need to enable it and set the search path when database initialization. You can configure it in your Cluster spec:
+> ```yaml
+> apiVersion: postgresql.cnpg.io/v1
+> kind: Cluster
+> spec:
+>   (...)
+>   bootstrap:
+>       initdb:
+>       postInitSQL:
+>           - ALTER SYSTEM SET search_path TO "$user", public, vectors;
+>           - CREATE EXTENSION IF NOT EXISTS "vectors";