Merge branch 'add-affinity-doc' into 'main'

add affinity documentation

See merge request !18

docs/affinity.md

+# Affinity
+
+* [kubernetes affinity ](https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/#affinity-and-anti-affinity)
+
+Pod constraints for istio operator is implemented following the kubernetes affinity documentation. Istio Operator can constraint a pod using nodeSelector or Affinity/Antti-affinity feature
+
+## nodeSelector 
+in the values.yaml  add nodeSelector like:
+```
+ nodeSelector:
+    <key>: <values> 
+```
+
+## nodeAffinity
+in the values of yaml. add nodeaffinity like:
+
+```
+affinity:
+    nodeAffinity:
+      requiredDuringSchedulingIgnoredDuringExecution:
+        nodeSelectorTerms:
+        - matchExpressions:
+          - key: kubernetes.io/e2e-az-name
+            operator: In
+            values:
+            - e2e-az1
+            - e2e-az2
+      preferredDuringSchedulingIgnoredDuringExecution:
+      - weight: 1
+        preference:
+          matchExpressions:
+          - key: another-node-label-key
+            operator: In
+            values:
+            - another-node-label-value
+```
+## podAffinity
+in the values.yaml add pod affinity/antiaffinity like:
+
+```
+affinity:
+    podAffinity:
+      requiredDuringSchedulingIgnoredDuringExecution:
+      - labelSelector:
+          matchExpressions:
+          - key: security
+            operator: In
+            values:
+            - S1
+        topologyKey: topology.kubernetes.io/zone
+    podAntiAffinity:
+      preferredDuringSchedulingIgnoredDuringExecution:
+      - weight: 100
+        podAffinityTerm:
+          labelSelector:
+            matchExpressions:
+            - key: security
+              operator: In
+              values:
+              - S2
+          topologyKey: topology.kubernetes.io/zone
+```
+
+
+