Document pgdog.role connection parameter

Author: levkk

docs/features/load-balancer/index.md

@@ -145,18 +145,67 @@ COMMIT;
 
 Read-only transactions are useful when queries depend on each other's results and need a consistent view of the database. Some Postgres database drivers allow this option to be set in the code, for example:
 
-=== "Golang/pgx"
-    ```golang
+=== "pgx (go)"
+    ```go
     tx, err := conn.BeginTx(ctx, pgx.TxOptions{
         AccessMode: pgx.ReadOnly,
     })
     ```
-=== "Node/Sequelize"
+=== "Sequelize (node)"
     ```javascript
     const tx = await sequelize.transaction({
       readOnly: true,
     });
     ```
+=== "SQLAlchemy (python)"
+    Add `postgresql_readonly=True` to [execution options](https://docs.sqlalchemy.org/en/20/core/connections.html#sqlalchemy.engine.Engine.execution_options), like so:
+    ```python
+    engine = create_engine("postgresql://user:pw@pgdog:6432/prod")
+              .execution_options(postgresql_readonly=True)
+    ```
+
+#### Primary-only connections
+
+If you need to override the load balancer routing decision and send a query (or all queries) to the primary, it's possible to do so by configuring the `pgdog.role` connection parameter.
+
+Configuring this connection parameter can be done at connection creation:
+
+=== "Connection URL"
+    ```bash
+    postgres://pgdog:pgdog@10.0.0.0:6432/database?options=-c%20pgdog.role%3Dprimary
+    ```
+=== "asyncpg (Python)"
+    ```python
+    conn = await asyncpg.connect(
+        user="pgdog",
+        password="pgdog",
+        database="pgdog",
+        host="10.0.0.0",
+        port=6432,
+        server_settings={
+            "pgdog.role": "primary",
+        }
+    )
+    ```
+=== "SQLAlchemy (Python)"
+    ```python
+    engine = create_async_engine(
+        "postgresql+asyncpg://pgdog:pgdog@10.0.0.0:6432/pgdog",
+        pool_size=20,
+        max_overflow=30,
+        pool_timeout=30,
+        pool_recycle=3600,
+        pool_pre_ping=True,
+        connect_args={"server_settings": {"pgdog.role": "primary"}},
+    )
+    ```
+
+The following values are supported:
+
+| Value | Routing decision |
+|-|-|
+| `primary` | Queries are sent to the primary database only. |
+| `replica` | Queries are load balanced between primary and replicas, depending on the value of the [`read_write_split`](../../configuration/pgdog.toml/general.md#read_write_split) setting. |
 
 
 ## Using the load balancer