Add custom scalar documentation

Resources/doc/definitions/custom-scalar.md

@@ -0,0 +1,53 @@
+# Custom scalar
+
+Here a simple example to add a custom scalar:
+
+```yaml
+DateTime:
+    type: custom-scalar
+    config:
+        serialize: ["AppBundle\\DateTimeType", "serialize"]
+        parseValue: ["AppBundle\\DateTimeType", "parseValue"]
+        parseLiteral: ["AppBundle\\DateTimeType", "parseLiteral"]
+```
+
+```php
+<?php
+
+namespace AppBundle;
+
+use GraphQL\Language\AST\Node;
+
+class DateTimeType
+{
+    /**
+     * @param \DateTime $value
+     *
+     * @return string
+     */
+    public static function serialize(\DateTime $value)
+    {
+        return $value->format('Y-m-d H:i:s');
+    }
+
+    /**
+     * @param mixed $value
+     *
+     * @return mixed
+     */
+    public static function parseValue($value)
+    {
+        return new \DateTime($value);
+    }
+
+    /**
+     * @param Node $valueNode
+     *
+     * @return string
+     */
+    public static function parseLiteral($valueNode)
+    {
+        return new \DateTime($valueNode->value);
+    }
+}
+```

Resources/doc/definitions/index.md

@@ -7,6 +7,7 @@ Definitions
 Go further 
 ----------
 
+* [Custom scalar](custom-scalar.md)
 * [Mutation](mutation.md)
 * [Relay](relay/index.md)
 * [Builders](builders/index.md)