Add shell completion

README.md

@@ -59,6 +59,29 @@ diple --repo owner/repository \
   --endpoint https://github.example.com/api/graphql
 ```
 
+## Shell completion
+
+`diple` generates completion scripts without contacting GitHub or loading the
+configuration. Choose the command for your shell:
+
+```sh
+# Bash: current session
+source <(diple completion bash)
+
+# Zsh: current session
+source <(diple completion zsh)
+
+# Fish: install for the current user
+diple completion fish > ~/.config/fish/completions/diple.fish
+```
+
+For persistent Bash completion, write the generated output to a directory
+loaded by your distribution's `bash-completion` package. For persistent Zsh
+completion, write it to a file named `_diple` in a directory on `$fpath`, then
+run `compinit`. `diple completion --help` lists the supported shells, while
+`diple --help` shows grouped command-line options, defaults, configuration
+precedence, and authentication behavior.
+
 ## Configuration
 
 The optional TOML configuration is loaded from