Add example use for password function (#126)

Feature introduced in brianc/node-postgres#1926.

content/features/1-connecting.mdx

@@ -90,6 +90,36 @@ client.query('SELECT NOW()', (err, res) => {
 })
 ```
 
+Many cloud providers include alternative methods for connecting to database instances using short-lived authentication tokens. node-postgres supports dynamic passwords via a callback function, either synchronous or asynchronous. The callback function must resolve to a string.
+
+```js
+const { Pool } = require('pg')
+const { RDS } = require('aws-sdk')
+
+const signerOptions = {
+  credentials: {
+    accessKeyId: 'YOUR-ACCESS-KEY',
+    secretAccessKey: 'YOUR-SECRET-ACCESS-KEY',
+  },
+  region: 'us-east-1',
+  hostname: 'example.aslfdewrlk.us-east-1.rds.amazonaws.com',
+  port: 5432,
+  username: 'api-user',
+}
+
+const signer = new RDS.Signer()
+
+const getPassword = () => signer.getAuthToken(signerOptions)
+
+const pool = new Pool({
+  host: signerOptions.hostname,
+  port: signerOptions.port,
+  user: signerOptions.username,
+  database: 'my-db',
+  password: getPassword,
+})
+```
+
 ### Programmatic Connection to Sockets
 
 Connections to unix sockets can also be made. This can be useful on distros like Ubuntu, where authentication is managed via the socket connection instead of a password.