Add note to docs about autocompletion

Author: brianshumate

website/source/docs/commands/index.html.md

@@ -79,6 +79,23 @@ Command Options
      Joins a server to another server in the WAN pool.
 ```
 
+## Autocompletion
+
+The `consul` command features opt-in subcommand autocompletion that you can
+enable for your shell with `consul -autocomplete-install`. After doing so,
+you can invoke a new shell and use the feature.
+
+For example, assume a tab is typed at the end of each prompt line:
+
+```
+$ consul e
+event  exec
+
+$ consul r
+raft    reload  rtt
+
+```
+
 ## Environment Variables
 
 In addition to CLI flags, Consul reads environment variables for behavior