docs: add description for FOLLOWER_POSTGRES_URI in Kubernetes environment variables documentation

Author: akshat5302

docs/self-hosting/methods/kubernetes.md

@@ -658,6 +658,7 @@ To configure the external secrets for your application, you need to define speci
 |                           | GITLAB_CLIENT_ID                  | Required if `services.silo.connectors.gitlab.enabled` is true          | GitLab client ID                                | your_gitlab_client_id                                                                                                                                                                 |
 |                           | GITLAB_CLIENT_SECRET              | Required if `services.silo.connectors.gitlab.enabled` is true          | GitLab client secret key                        | your_gitlab_client_secret_key                                                                                                                                                         |
 | pi_api_env_existingSecret | PLANE_PI_DATABASE_URL             | Required if `services.pi.enabled=true`                                 | PostgreSQL connection URL for Plane AI database | **k8s service example**: `postgresql://plane:plane@plane-pgdb.plane-ns.svc.cluster.local/plane_pi` <br> <br>**external**: `postgresql://username:password@your-db-host:5432/plane_pi` |
+|                           | FOLLOWER_POSTGRES_URI            | No                                                                     | Connection string for a PostgreSQL read replica | Same as DATABASE_URL. Used for read-heavy operations to reduce load on the primary database. **k8s**: `postgresql://plane:plane@plane-pgdb.plane-ns.svc.cluster.local:5432/plane`     |
 |                           | AMQP_URL                          | Required if `services.pi.enabled=true`                                 | RabbitMQ connection URL                         | **k8s service example**: `amqp://plane:plane@plane-rabbitmq.plane-ns.svc.cluster.local:5672/` <br> <br> **external**: `amqp://username:password@your-rabbitmq-host:5672/`             |
 |                           | AES_SECRET_KEY                    | Required if `services.pi.enabled=true`                                 | AES secret key for Plane AI                     | dsOdt7YrvxsTIFJ37pOaEVvLxN8KGBCr (or your own value)                                                                                                                                  |
 |                           | OPENAI_API_KEY                    | required if `services.pi.ai_providers.openai.enabled` is true          | OpenAI API key                                  | your_openai_api_key                                                                                                                                                                   |