Update charts/docker-mailserver/README.md ToC

Updates the Table of Contents for the chart's README.md with the appended sections.

Author: j-heffron

charts/docker-mailserver/README.md

@@ -15,6 +15,9 @@
   - [Certificate](#certificate)
 - [Ports](#ports)
 - [Persistence](#persistence)
+  - [Backing Storage](#backing-storage)
+    - [Generic / All](#generic--all)
+    - [NFS](#nfs)
 - [Upgrading to Version 3.0.0](#upgrading-to-version-3)
 - [Development](#development)
   - [Testing](#testing)