Update docs with CKEditor configuration

Author: em411

doc/installation.md

@@ -29,6 +29,7 @@ Since FOSCKEditorBundle 2.0, to make Twig render the WYSIWYG editor, you must ad
 twig:
     form_themes:
         - '@FOSCKEditor/Form/ckeditor_widget.html.twig'
+        - '@BitBagSyliusCmsPlugin/Form/ckeditor_widget.html.twig'
 ```
 
 Import required config in your `config/packages/_sylius.yaml` file:

doc/wysiwyg.md

@@ -2,7 +2,7 @@
 
 This plugin comes with a default [FOSCKEditorBundle](https://github.com/FriendsOfSymfony/FOSCKEditorBundle) integration.
 It has also been customized to use image upload feature. Every time you upload an image in the CKEditor, a new image media
-is being created. 
+is being created.
 
 ## General usage
 
@@ -12,3 +12,24 @@ Take [the BlockTranslationType](../src/Form/Type/Translation/BlockTranslationTyp
 **Note:**
 
 *In the WYSIWYG fields, you can use Twig function nesting. Read more [here](twig-functions-in-admin.md).*
+
+## Configuration
+
+If you want to customize any behavior of the CKEditor, you have to override one of these files: [CKEditor config](../src/Resources/config/fos_ck_editor/fos_ck_editor.yml) or [CKEditor js config](../src/Resources/views/Form/ckeditor_widget.html.twig)
+
+**Useful links:**
+- [CKEditor configuration](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_config.html)
+- [CKEditor DTD configuration](https://ckeditor.com/docs/ckeditor4/latest/api/CKEDITOR_dtd.html)
+
+
+**Note:**
+
+Remember to add your js configuration to `config/packages/twig.yaml` file:**
+```yaml
+# Symfony 2/3: app/config/config.yml
+# Symfony 4: config/packages/twig.yaml
+
+twig:
+    form_themes:
+        - 'path to your configuration'
+```