add facebook login example (#18)

docs/examples/facebook_login.md

@@ -0,0 +1,67 @@
+# Facebook Login Example
+
+This example demonstrates how to implement Facebook Login using the Laravel Facebook Graph SDK, based on the original example for [Facebook SDK V6 for PHP](https://github.com/joelbutcher/facebook-php-graph-sdk/blob/6.x/docs/examples/facebook_login.md).
+
+## Example
+
+In this example, we will be using two routes: one to generate the Facebook Login URL and one for the callback where Facebook will redirect the user after the login dialog is called.
+
+> Before we start, make sure that you have added the  **Persistent Data Handlers** in the `AppServiceProvider`, You can find an example of how to do this [here](./docs/examples/persistent_data_storage.md)
+
+## Route for generating Facebook Login URL
+
+```php
+
+use JoelButcher\Facebook\Facades\Facebook as FacebookFacade;
+
+...
+
+Route::get('facebook/login', function () {
+    // set the permissions (scopes)
+    // by default the email and public_profile permission are added
+    // in the HandlesAuthentication trait
+
+    $scopes = ['pages_manage_posts', 'pages_read_engagement', 'pages_show_list'];
+
+    $loginUrl = FacebookFacade::getRedirect(route('facebook.callback'), $scopes);
+
+    echo '<a href="' . $loginUrl . '">Log in with Facebook!</a>';
+});
+```
+
+## Route for Facebook callback
+
+```php
+use JoelButcher\Facebook\Facebook;
+use JoelButcher\Facebook\Facades\Facebook as FacebookFacade;
+
+...
+
+Route::get('facebook/callback', function () {
+    // this is how to get the token
+    // make sur to save the token in the database
+    // because you can't use this methods again unless 
+    // you repeat the long proccess
+
+    $token = FacebookFacade::getToken();
+
+
+    // this is how to use the token
+
+    $fb = app(Facebook::class);
+    $fb->getFacebook()->setDefaultAccessToken($token);
+
+    return $fb->getUser();
+})->name('facebook.callback');
+
+```
+
+This will return a JSON object with the user's name and ID:
+
+
+```json
+{
+    "name":"Facebook User",
+    "id":"XXXXXXXXXXXXXXXX"
+}
+```