Add UPGRADE file

UPGRADE.md

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+UPGRADE FROM 2.1.x TO 2.2.x
+===========================
+
+Upgrading your capifony configuration from **2.1.x** to **2.2.x** is quite easy.
+If you don't have a custom `Capfile` file, delete it and recreate it using the
+following command:
+
+    capifony .
+
+Important: This is mandatory for both symfony 1.x and Symfony2 projects.
+
+If you have a custom `Capfile` file, you just have to change the way you load
+capifony. In capifony 2.1.x and for Symfony2 projects, the line below was used
+to load capifony:
+
+    load Gem.find_files('symfony2.rb').first.to_s
+
+You have to replace this line by the following one:
+
+    require 'capifony_symfony2'
+
+For symfony 1.x projects, the line below was used for the same purpose:
+
+    load Gem.find_files('symfony1.rb').first.to_s
+
+You have to replace it by the following line:
+
+    load Gem.find_files('capifony_symfony1.rb').first.to_s
+
+That's all.