Add docs for using Helm `--set` to create extraConfig variables (#5474)

* Add docs for using Helm `--set` to create extraConfig variables

Based on info provided for `consul-helm` issue 74.

* fixing formatting

website/source/docs/platform/k8s/helm.html.md

@@ -136,6 +136,11 @@ and consider if they're appropriate for your deployment.
             "log_level": "DEBUG"
           }
         ```
+        This can also be set using Helm's `--set` flag (consul-helm v0.7.0 and later), using the following syntax:
+
+        ```shell
+        --set 'server.extraConfig="{"log_level": "DEBUG"}"'
+        ```
 
   * <a name="v-server-extravolumes" href="#v-server-extravolumes">`extraVolumes`</a> (`array: []`) - A list of extra volumes to mount for server agents. This is useful for bringing in extra data that can be referenced by other configurations at a well known path, such as TLS certificates or Gossip encryption keys. The value of this should be a list of objects. Each object supports the following keys:
 
@@ -206,6 +211,11 @@ and consider if they're appropriate for your deployment.
             "log_level": "DEBUG"
           }
         ```
+        This can also be set using Helm's `--set` flag (consul-helm v0.7.0 and later), using the following syntax:
+
+        ```shell
+        --set 'client.extraConfig="{"log_level": "DEBUG"}"'
+        ```
 
   * <a name="v-client-extravolumes" href="#v-client-extravolumes">`extraVolumes`</a> (`array: []`) - A list of extra volumes to mount for client agents. This is useful for bringing in extra data that can be referenced by other configurations at a well known path, such as TLS certificates or Gossip encryption keys. The value of this should be a list of objects. Each object supports the following keys: