Add Immutable flag description to flags reference

Author: vienthuong

resources/references/core-reference/dal-reference/flags-reference.md

@@ -26,3 +26,5 @@ nav:
 | SetNullOnDelete | In case the referenced association data will be deleted, the related data will be set to null and an Written event will be thrown                                                                                                                                                                                                               |
 | Since | The "Since" flag defines since which Shopware version the field is available.                                                                                                                                                                                                                                                                   |
 | WriteProtected | By setting the "WriteProtected" flag, write access via API can be restricted. This flag is mostly used to protect indexed data from direct writing via API.                                                                                                                                                                                     |
+| Immutable | By setting the "Immutable" flag, it indicates that the field is write-once and then read-only                                                                                                                                                                                     |
+