add transformer documentation

Resources/doc/transformers.md

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+## Creating your own transformers
+
+Transformers are used to change the html data before it is saved.
+
+For an example transformer see [Transformer/StripJS.php](Transformer/StripJS.php)
+
+### Step 1: Create your transformer
+
+src/AcmeDemoBundle/Transformer/RemoveImages.php
+
+```php
+<?php
+
+namespace Acme\DemoBundle\Transformer;
+
+use Symfony\Component\Form\DataTransformerInterface;
+
+class RemoveImages implements DataTransformerInterface
+{
+    /**
+     * {@inheritDoc}
+     */
+    public function transform($data)
+    {
+        return $data;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    public function reverseTransform($data)
+    {
+        return preg_replace('/<img[^>]+\>/is', '', $data);
+    }
+}
+```
+
+### Step 2: Tag your transformer so it can be detected.
+
+src/AcmeDemoBundle/config/services.yml
+
+```yaml
+    acme_demo_ckeditor.transformer.strip_comments:
+        class: Acme\DemoBundle\Transformer\RemoveImages
+        tags:
+            - { name: trsteel_ckeditor.transformer, alias: remove_images }
+```
+
+Note: The alias must be unique.
+
+### Step 3: Enable your custom transformer
+
+Enable the transformer globally in config.yml
+
+```yaml
+trsteel_ckeditor:
+    transformers: ['remove_images']
+```
+
+Or enable it on a specific form
+
+```php
+<?php
+
+$form = $this->createFormBuilder($post)
+            ->add('title', 'text')
+            ->add('content', 'ckeditor', array(
+                'transformers' => array('remove_images'),
+            ))
+            ->getForm()
+;
+```
+
+Note: If you override the transformers value none of the default transformers will be including (eg strip_js, strip_css etc)