Add documentation for GERRIT_KEY_PREFIX

Change-Id: Ic4bfc24b605a625cf1c27bfb5a471017d9981d78
diff --git a/dual-master/README.md b/dual-master/README.md
index 8ac1bdc..5e2dbcd 100644
--- a/dual-master/README.md
+++ b/dual-master/README.md
@@ -67,6 +67,8 @@
 * `MASTER1_SUBDOMAIN`: Optional. Name of the master 1 sub domain. `gerrit-master-1-demo` by default.
 * `MASTER2_SUBDOMAIN`: Optional. Name of the master 2 sub domain. `gerrit-master-2-demo` by default.
 * `CLUSTER_DESIRED_CAPACITY`: Optional.  Number of EC2 instances composing the cluster. `1` by default.
+*  GERRIT_KEY_PREFIX : Optional. Secrets prefix used during the [Import into AWS Secret Manager](#import-into-aws-secret-manager).
+  `gerrit_secret` by default.
 
 ### Prerequisites
 
@@ -127,7 +129,9 @@
 
 You can now run the [script](../gerrit/add_secrets_aws_secrets_manager.sh) to
 upload them to AWS Secret Manager:
-`add_secrets_aws_secrets_manager.sh /path/to/your/keys/directory`
+`add_secrets_aws_secrets_manager.sh /path/to/your/keys/directory secret_prefix`
+
+When `secret_prefix` is omitted, it is set to `gerrit_secret` by default.
 
 ### Publish custom Gerrit Docker image
 
diff --git a/master-slave/README.md b/master-slave/README.md
index 19c704e..e12ea3a 100644
--- a/master-slave/README.md
+++ b/master-slave/README.md
@@ -73,6 +73,8 @@
 * `HOSTED_ZONE_NAME`: Optional. Name of the hosted zone. `mycompany.com` by default.
 * `MASTER_SUBDOMAIN`: Optional. Name of the master sub domain. `gerrit-master-demo` by default.
 * `SLAVE_SUBDOMAIN`: Optional. Name of the slave sub domain. `gerrit-slave-demo` by default.
+*  GERRIT_KEY_PREFIX : Optional. Secrets prefix used during the [Import into AWS Secret Manager](#import-into-aws-secret-manager).
+  `gerrit_secret` by default.
 * `CLUSTER_DESIRED_CAPACITY`: Optional.  Number of EC2 instances composing the cluster. `1` by default.
 
 *NOTE: if you are planning to run the monitoring stack, set the
@@ -140,7 +142,9 @@
 
 You can now run the [script](../gerrit/add_secrets_aws_secrets_manager.sh) to
 upload them to AWS Secret Manager:
-`add_secrets_aws_secrets_manager.sh /path/to/your/keys/directory`
+`add_secrets_aws_secrets_manager.sh /path/to/your/keys/directory secret_prefix`
+
+When `secret_prefix` is omitted, it is set to `gerrit_secret` by default.
 
 ### Publish custom Gerrit Docker image
 
diff --git a/single-master/README.md b/single-master/README.md
index e87da8b..cc04708 100644
--- a/single-master/README.md
+++ b/single-master/README.md
@@ -70,6 +70,8 @@
 * `DNS_ROUTING_STACK_NAME`: Optional. Name of the DNS routing stack. `gerrit-dns-routing` by default.
 * `HOSTED_ZONE_NAME`: Optional. Name of the hosted zone. `mycompany.com` by default.
 * `SUBDOMAIN`: Optional. Name of the sub domain. `gerrit-master-demo` by default.
+*  GERRIT_KEY_PREFIX : Optional. Secrets prefix used during the [Import into AWS Secret Manager](#import-into-aws-secret-manager).
+  `gerrit_secret` by default.
 
 ### Prerequisites