#145 add TrimTransformer documentation

Author: xmarchegay

docs/index.md

@@ -123,7 +123,7 @@
             - [ImplodeTransformer]
             - [SlugifyTransformer]
             - [SprintfTransformer]
-            - [TrimTransformer]
+            - [TrimTransformer](reference/transformers/trim_transformer.md)
         - XML
             - [XpathEvaluatorTransformer](reference/transformers/xpath_evaluator.md)
     - Other bridges

docs/reference/transformers/trim_transformer.md

@@ -0,0 +1,32 @@
+TrimTransformer
+=========================
+
+Strip whitespace (or other characters) from the beginning and end of a string
+
+This transformer uses the php internal function: https://www.php.net/manual/en/function.trim.php
+
+Task reference
+--------------
+
+* **Service**: `CleverAge\ProcessBundle\Transformer\String\TrimTransformer`
+* **Transformer code**: `trim`
+
+Accepted inputs
+---------------
+
+Any value that can be cast to string and null.
+
+Possible outputs
+----------------
+
+Depending on the input :
+- `null` if the input is null
+- `string` if the input is not null
+
+Options
+-------
+
+| Code | Type | Required | Default               | Description                                                                                                                                                         |
+| ---- | ---- | :------: |-----------------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `charlist` | `string` | | ***" \t\n\r\0\x0B"*** | List of characters to trim |
+