Document installation instructions

docs/reference/installation_instructions.rst

@@ -1,2 +1,23 @@
 Installation instructions
 =========================
+
+To install Site API first add it as a dependency to your project:
+
+```sh
+
+    composer require netgen/ezplatform-site-api:^2.5
+
+Once Site API is installed, activate the bundle in ``app/AppKernel.php`` file by adding it to the
+``$bundles`` array in ``registerBundles()`` method, together with other required bundles:
+
+```php
+
+    public function registerBundles()
+    {
+        ...
+
+        $bundles[] = new Netgen\Bundle\EzPlatformSiteApiBundle\NetgenEzPlatformSiteApiBundle();
+        $bundles[] = new Netgen\Bundle\EzPlatformSearchExtraBundle\NetgenEzPlatformSearchExtraBundle;
+
+        return $bundles;
+    }