Move "Message placeholder conversion" documentation

Signed-off-by: Henrique Moody <henriquemoody@gmail.com>

docs/feature-guide.md

@@ -228,26 +228,6 @@ Array
 Note that `getMessage()` will only return a message when the specific validation
 in the chain fails.
 
-## Message placeholder conversion
-
-Message in Validation usually have placeholders that are in between "{{" and
-"}}" characters. To replace those placeholders with the real parameters, we need
-to convert them to string.
-
-We use the `ParameterStringifier` to convert those parameters into a string.
-Our default implementation will convert all parameters with
-[Respect\Stringifier](https://github.com/Respect/Stringifier) unless the
-parameter is called `name` and it is already a string.
-
-It is possible to overwrite that behavior by creating a custom implementation of
-the `ParameterStringifier` and passing it to the `Factory`:
-
-```php
-Factory::setDefaultInstance(
-    (new Factory())->withParameterStringifier(new MyCustomStringifier())
-);
-```
-
 ## Validator name
 
 On `v::attribute()` and `v::key()`, `{{name}}` is the attribute/key name. For others,

docs/message-placeholder-conversion.md

@@ -0,0 +1,19 @@
+## Message placeholder conversion
+
+Messages in Validation usually have placeholders that are in between "{{" and
+"}}" characters. To replace those placeholders with the real parameters, we need
+to convert them to string.
+
+We use the `ParameterStringifier` to convert those parameters into a string.
+Our default implementation will convert all parameters with
+[Respect\Stringifier](https://github.com/Respect/Stringifier) unless the
+parameter is called `name` and it is already a string.
+
+It is possible to overwrite that behavior by creating a custom implementation of
+the `ParameterStringifier` and passing it to the `Factory`:
+
+```php
+Factory::setDefaultInstance(
+    (new Factory())->withParameterStringifier(new MyCustomStringifier())
+);
+```