Add documentation

docs/add-ons/external-secrets.md

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+
+# External Secrets Operator
+
+[External Secrets Operator](https://external-secrets.io/latest) is a Kubernetes operator that integrates external secret management systems like AWS Secrets Manager, HashiCorp Vault, Google Secrets Manager, Azure Key Vault and many more. The operator reads information from external APIs and automatically injects the values into a Kubernetes Secret.
+
+## Usage
+
+The External Secrets Operator can be deployed by enabling the add-on via the following.
+
+```hcl
+enable_external_secrets = true
+```
+
+You can optionally customize the Helm chart that deploys the operator via the following configuration.
+
+```hcl
+  enable_external_secrets = true
+  external_secrets_helm_config = {
+    name                       = "external-secrets"
+    chart                      = "external-secrets"
+    repository                 = "https://charts.external-secrets.io/"
+    version                    = "0.5.6"
+    namespace                  = "external-secrets"
+  }
+```
+
+###  GitOps Configuration
+
+The following properties are made available for use when managing the add-on via GitOps.
+
+Refer to [locals.tf](modules/kubernetes-addons/external-secrets/locals.tf) for latest config. GitOps with ArgoCD Add-on repo is located [here](https://github.com/aws-samples/eks-blueprints-add-ons/blob/main/chart/values.yaml).
+
+```hcl
+  argocd_gitops_config = {
+    enable = true
+  }
+```