added an UPDATE.md file to explain how to upgrade

UPDATE.md

@@ -0,0 +1,53 @@
+How to update your project?
+===========================
+
+This document explains how to upgrade from one Symfony2 PR version to the next
+one. It only discusses changes that need to be done when using the "public"
+API of the framework. If you "hack" the core, you should probably follow the
+timeline closely anyway.
+
+PR8 to PR9
+----------
+
+* `Symfony\Bundle\FrameworkBundle\Util\Filesystem` has been moved to
+  `Symfony\Component\HttpKernel\Util\Filesystem`
+
+* The `Execute` constraint has been renamed to `Callback`
+
+* The HTTP exceptions classes signatures have changed:
+
+    Before: throw new NotFoundHttpException('Not Found', $message, 0, $e);
+    After:  throw new NotFoundHttpException($message, $e);
+
+* The RequestMatcher class does not add `^` and `$` anymore to regexp.
+
+    You need to update your security configuration accordingly for instance:
+
+    Before:
+
+        profiler:
+            pattern:  /_profiler/.*
+
+    After:
+
+        profiler:
+            pattern:  ^/_profiler
+
+* Global templates under `app/` moved to a new location (old directory did not
+  work anyway):
+
+    Before:
+        app/views/base.html.twig
+        app/views/AcmeDemoBundle/base.html.twig
+
+    After:
+        app/Resources/views/base.html.twig
+        app/AcmeDemoBundle/Resources/views/base.html.twig
+
+* Namespace for validators has changed from `validation` to `assert`:
+
+    Before: @validation:NotNull
+    After:  @assert:NotNull
+
+    Moreover, the `Assert` prefix used for some constraints has been removed
+    (`AssertTrue` to `True`).