[ci skip] Make it clear that PHP <5.4 usage is discouraged
Related: #3450
diff --git a/readme.rst b/readme.rst
index c329614..dd59fd8 100644
--- a/readme.rst
+++ b/readme.rst
@@ -29,7 +29,11 @@
Server Requirements
*******************
-- PHP version 5.2.4 or newer.
+PHP version 5.4 or newer is recommended.
+
+It should work on 5.2.4 as well, but we strongly advise you NOT to run
+such old versions of PHP, because of potential security and performance
+issues, as well as missing features.
************
Installation