Add FEATURE_GATES to ConfigMap documentation (#2538)

Issue #, if available: #2537

Description of changes:

Added FEATURE_GATES to ConfigMap documentation, because it is required as described in the issue #2537 


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Author: jstrahle

docs/content/docs/user-docs/openshift.md

@@ -76,6 +76,7 @@ ENABLE_LEADER_ELECTION=true
 LEADER_ELECTION_NAMESPACE=
 RECONCILE_DEFAULT_MAX_CONCURRENT_SYNCS=1
 FEATURE_FLAGS=
+FEATURE_GATES=
 ```
 
 Now use `config.txt` to create a `ConfigMap` in your OpenShift cluster: