Add session affinity FAQ (#299)

* Add session affinity FAQ
---------

Co-authored-by: Chengjun Fu <cfu23@apple.com>

doc/FAQ.md

@@ -1,13 +1,17 @@
 FAQ - Frequently Asked Questions
-======
+================================
 
 Readyness probe fails
----------------
+---------------------
 
-When encountering errors like `Readiness probe failed: Node not found with CONNECTED state` or `Multi-Attach error for volume "pvc-xxxxxx-xxx-xxx-xxxx-xxxxxxxxx" Volume is already exclusively attached to one node and can't be attached to another`, it means Kubernetes can't provide the pod access to the persistent data it wants. 
+When encountering errors like `Readiness probe failed: Node not found with CONNECTED state` or `Multi-Attach error for volume "pvc-xxxxxx-xxx-xxx-xxxx-xxxxxxxxx" Volume is already exclusively attached to one node and can't be attached to another`, it means Kubernetes can't provide the pod access to the persistent data it wants.
 
-When this happens, reach out to your Kubernetes cluster administrators to find and fix the problem manually. 
+When this happens, reach out to your Kubernetes cluster administrators to find and fix the problem manually.
 
 For more background, see https://blog.mayadata.io/recover-from-volume-multi-attach-error-in-on-prem-kubernetes-clusters
 
-(see https://github.com/cetic/helm-nifi/issues/47#issuecomment-1122702262)
+(see https://github.com/cetic/helm-nifi/issues/47#issuecomment-1122702262)
+
+## Session Afffinity
+
+As mentioned in the official NIFI document regarding [session affinity](https://nifi.apache.org/docs/nifi-docs/html/administration-guide.html#session_affinity), it's required to implement this feature for your ingress. Please refer to the ingress controller your are using for how to implement it. One example for GKE is with [issue #271](https://github.com/cetic/helm-nifi/issues/271). If NIFI cluster has more than one node, the session affinity has to be there due to the stateful implementation of each node.