Add code to enable autocomplete so users can copy it

docs/adguard-vpn-for-linux/autocomplete.md

@@ -9,6 +9,12 @@ To enable it, see the bash-completion hint that is shown after installing or upd
 
 ![bash-completion hint](https://cdn.adguard-vpn.com/blog/new/6x3djbash-completion-hint.png)
 
+If you did not enable completions during installation and would like to do so later, edit your shell configuration file, typically located in your home directory such as `~/.bashrc` or `~/.zshrc`, and add the following line. You will need to update the path if you chose a different output directory during installation.
+
+```sh
+[ -s "/opt/adguardvpn_cli/bash-completion.sh" ] && \. "/opt/adguardvpn_cli/bash-completion.sh"
+```
+
 To use the feature, just start typing the command you want and hit the Tab ↹ key — the command will automatically complete with a necessary word or will show a choice of completion options.
 
 ![bash-completion locations command](https://cdn.adguard-vpn.com/blog/new/1g4nhVPN-CLI-autocomplete.png)