Updated upgrade instructions.

UPGRADE-2.1.md

@@ -425,6 +425,9 @@
     (or any other of the BIND events). In case you used the CallbackValidator
     class, you should now pass the callback directly to `addEventListener`.
 
+  * Core translation messages are changed. Dot is added at the end of each message.
+    Overwritten core translations should be fixed if any. More info [here](https://github.com/symfony/symfony/issues/3379).
+
 ### Session
 
   * Flash messages now return an array based on their type. The old method is