Add upgrade instructions

Author: GromNaN

UPGRADE-2.16.md

@@ -0,0 +1,10 @@
+# UPGRADE FROM 2.15 to 2.16
+
+## Package `doctrine/cache`
+
+The package `doctrine/cache` no longer required.
+
+If you use `Doctrine\ODM\MongoDB\Configuration::getMetadataCacheImpl()`,
+then you need to require `doctrine/cache` explicitly in your `composer.json`;
+or use `Doctrine\ODM\MongoDB\Configuration::getMetadataCache()` instead.
+