Added documentation for using translation

Author: emsifa

README.md

@@ -267,6 +267,38 @@ $validation_a = $validator->make($dataset_a, [
 $validation_a->validate();
 ```
 
+## Translation
+
+Translation is different with custom messages. 
+Translation needed when you use custom message for rule `in`, `not_in`, `mimes`, and `uploadede_file`.
+
+For example if you use rule `in:1,2,3` we will set invalid message like "The Attribute only allows '1', '2', or '3'" 
+where message "'1', '2', or '3'" is comes from ":allowed_values" tag.
+So if you have custom Indonesian message ":attribute hanya mengizinkan :allowed_values", 
+we will set invalid message like "Attribute hanya mengizinkan '1', '2', or '3'" which is the "or" word is not Indonesian language.
+
+So, to solves this problem, we can use translation like this:
+
+```php
+// Set translation for words 'and' and 'or'.
+$validator->setTranslations([
+    'and' => 'dan',
+    'or' => 'atau'
+]);
+
+// Set custom message for 'in' rule
+$validator->setMessage('in', ":attribute hanya mengizinkan :allowed_values");
+
+// Validate
+$validation = $validator->validate($inputs, [
+    'nomor' => 'in:1,2,3'
+]);
+
+$message = $validation->errors()->first('nomor'); // "Nomor hanya mengizinkan '1', '2', atau '3'" 
+```
+
+> Actually, our built-in rules only use words 'and' and 'or' that you may need to translates.
+
 ## Working with Error Message
 
 Errors messages are collected in `Rakit\Validation\ErrorBag` object that you can get it using `errors()` method.