Add sequence group example to documentation (#2)

* Add sequence group example to documentation

* Update Readme.md

Author: aradoje

Readme.md

@@ -21,10 +21,9 @@ final class EmailUser
 {
     /**
      * @Assert\NotBlank
-     * @Assert\Uuid
      * @EntityExist(entity="App\Entity\User")
      *
-     * @var string UUID to user's id property
+     * @var int User's id property
      */
     private $user;
 
@@ -39,6 +38,31 @@ final class EmailUser
     // ...
 ```
 
+In case you are using other constraints to validate the property before entity should be checked in the database (like `@Assert\Uuid`) you should use [Group sequence](https://symfony.com/doc/current/validation/sequence_provider.html) in order to avoid 500 errors from Doctrine mapping.
+
+```php
+namespace App\Message\Command;
+
+use Happyr\Validator\Constraint\EntityExist;
+use Symfony\Component\Validator\Constraints as Assert;
+
+/**
+ * @Assert\GroupSequence({"EmailUser", "DatabaseCall"})
+ */
+final class EmailUser
+{
+    /**
+     * @Assert\NotBlank
+     * @Assert\Uuid
+     * @EntityExist(entity="App\Entity\User", groups={"DatabaseCall"}, property="uuid")
+     *
+     * @var string Uuid
+     */
+    private $user;
+
+    // ...
+```
+
 ## Install
 
 ```console
@@ -59,4 +83,4 @@ services:
 ## Note
 
 The Validator will not produce a violation when value is empty. This means that you should most likely use it in
-combination with `NotBlank`. 
+combination with `NotBlank`.