doc: update readme file.

Author: khaledalam

README.md

@@ -34,9 +34,12 @@ composer require khaledalam/unit
 
 ```php
 <?php
+// test.php
 
 // __construct(string $name, Name|string $symbol, float $factor, Dimension $dimension)
 
+require __DIR__ . '/vendor/autoload.php';
+
 use KhaledAlam\Unit\Unit;
 use KhaledAlam\Unit\Name;
 use KhaledAlam\Unit\Dimension;
@@ -59,6 +62,11 @@ echo $sum; // "3 m"
 
 ?>
 ```
+output:
+```bash
+% php main.php
+3 m
+```
 
 ---
 

src/Unit/UnitRegistry.php

@@ -4,7 +4,7 @@
 
 use KhaledAlam\Unit\Unit;
 
-class UnitRegistry
+final class UnitRegistry
 {
     /** @var array<string, Unit> */
     private static array $units = [];