README: add missing CONFIG_MEMCG kernel config option for oomd

We need to enable this otherwise systemd-oomd.service fails to start.

Fixes:
ConditionControlGroupController=memory was not met

(cherry picked from commit aa329b8)
(cherry picked from commit a50e6c5)
(cherry picked from commit 1817431)

Author: jameshilliard

README

@@ -162,6 +162,7 @@ REQUIREMENTS:
 
         Required for systemd-oomd:
           CONFIG_PSI
+          CONFIG_MEMCG
 
         Note that kernel auditing is broken when used with systemd's container
         code. When using systemd in conjunction with containers, please make