feat(upgrade): add an upgrade guide

Author: lanfisis

UPGRADE-2.0.md

@@ -0,0 +1,20 @@
+# UPGRADE FROM `1.2` TO `2.0`
+
+**Replace form type option name**
+
+If you use one of the following options with Media Manager form type (`AudioType`, `FaviconType`, `ImageType`, `PdfType` or `VideoType`), 
+you had to rename it as follows:
+
+* `["file-type" => "TYPE"]` became `["file_type" => "TYPE"]`
+* `["filter-width" => INT]` became `["filter_width" => INT]]`
+
+**Use new services**
+
+`FileHelper` service has been split into dedicated services:
+* `FileRepositoryInterface` to list file from a folder or get one file by it path
+* `DirectoryOperatorInterface` where all directory operations takes place
+* `FileMetadataResolverInterface` for file types resolution
+* `FilePathResolverInterface` for absolute and relative path resolution
+* `FileValidatorInterface` for file validation
+
+If you use `FileHelper` service, you need to inject one of the service above instead.