Merge pull request #2472 from greg0ire/warn-about-signature-incompatibility

Add warning about potential signature incompatibility

Author: malarzm

UPGRADE-2.5.md

@@ -4,3 +4,10 @@
 
 * MongoDB ODM 2.5 requires PHP 7.4 or newer. If you're not running PHP 7.4 yet,
   it's recommended that you upgrade to PHP 7.4 before upgrading ODM.
+
+## `doctrine/collections` v2 compatibility
+
+ODM supports doctrine/collections 2.x along 1.x. Please note that we added
+`findFirst(...)` and `reduce(...)` methods to comply with the new `Collection`
+interface. Make sure signatures of new methods comply with your own ones,
+should you have implemented such methods before.