docs: mention explicitly about pool.destroy() (#77)

README.md

@@ -33,6 +33,10 @@ const pool = new Tinypool({
 })
 const result = await pool.run({ a: 4, b: 6 })
 console.log(result) // Prints 10
+
+// Make sure to destroy pool once it's not needed anymore
+// This terminates all pool's idle workers
+await pool.destroy()
 ```
 
 ```js