Create pull request template with deprecation info

This template should make it clear to every contributor,
that this repository and the bundle became deprecated.

Author: SenseException

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,5 @@
+## This bundle became deprecated
+
+Please be aware that this bundle became deprecated and won't be compatible with Symfony 5 and upwards.
+Read more about the deprecation and the alternatives for this bundle in the issue 
+[#156](https://github.com/doctrine/DoctrineCacheBundle/issues/156).