Provide documentation for SymfonyExtension v2

docs/configuration_reference.md

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+## Configuration reference
+
+By default, if no confguration is passed, *SymfonyExtension* will try its best to guess it.
+The full configuration tree looks like that:
+
+```yaml
+# behat.yml.dist / behat.yml
+
+default:
+    extensions:
+        FriendsOfBehat\SymfonyExtension:
+            bootstrap: ~
+            kernel:
+                class: ~
+                file: ~
+                environment: ~
+                debug: ~
+```
+
+ * **`bootstrap`**: 
+ 
+    It is a path to the file requried once while the extension is loaded. You can use this file to set up your testing 
+    environment - set some enviornment variables or preload an external file.
+    If you do not pass any, it would look for either `config/bootstrap.php` (Symfony 4) or `app/autoload.php` (Symfony 3). 
+    If none are found, no file would be loaded.
+    
+ * **`kernel.class`**:
+ 
+    It is a fully qualified class name of the application kernel class.
+    If you do not pass any, it would look for either `App\Kernel` (Symfony 4) or `AppKernel` (Symfony 3).
+    If none are found, an exception would be thrown and you would be required to specify it explicitly.
+    
+ * **`kernel.file`**:
+ 
+    It is a path to the file containing the application kernel class. You might want to set it if your kernel is not
+    autoloaded by Composer's autoloaded.
+    If `kernel.class` is not defined, it would automatically use `app/AppKernel.php` if `AppKernel` class was autoconfigured.
+    
+ * **`kernel.environment`**:
+ 
+    It allows you to force using a given environment. If it is not set, it uses `APP_ENV` environment variable if defined
+    or falls back to `test`.
+    
+ * **`kernel.debug`**:
+ 
+    It allows you to force enabling or disabling debug mode. If it is not set, it uses `APP_DEBUG` environment variable 
+    if defined or falls back to `true`.