Document shell completion in README

README.md

@@ -104,6 +104,7 @@ vaultik [--config <path>] info
 vaultik [--config <path>] remote info [--json]
 vaultik [--config <path>] store info
 vaultik [--config <path>] database purge [--force]
+vaultik completion <bash|zsh|fish|powershell>
 vaultik version
 ```
 
@@ -120,6 +121,25 @@ vaultik version
 * `VAULTIK_CONFIG`: Path to config file (overridden by `--config`)
 * `VAULTIK_INDEX_PATH`: Override local SQLite index path
 
+### shell completion
+
+```sh
+# zsh: load for the current session
+source <(vaultik completion zsh)
+
+# zsh: install permanently
+vaultik completion zsh > "${fpath[1]}/_vaultik"
+
+# bash: load for the current session
+source <(vaultik completion bash)
+
+# bash: install permanently (Linux)
+vaultik completion bash > /etc/bash_completion.d/vaultik
+
+# fish
+vaultik completion fish > ~/.config/fish/completions/vaultik.fish
+```
+
 ### command details
 
 **`config init`**: Write a default config file with commented explanations for