feature: bash completes the vpn-* commands, their options and the names they take.

README.md

@@ -355,6 +355,23 @@ indented, with mode 0600, and reminds that `vpn-router-setup` applies it. The st
 writes the file the same way, and a lock keeps the two from overwriting each other's change. A
 refused request exits with status 2, any other failure with 1.
 
+## Bash completion
+
+Bash completes the commands, options and arguments of `vpn-router-user`, `vpn-router-group`,
+`vpn-router-token`, `vpn-config-edit`, `vpn-router-backup`, `vpn-router-ca` and
+`vpn-router-caddy` with the Tab key. The package installs a file per command in
+`/usr/share/bash-completion/completions`, which the `bash-completion` package, recommended by
+`vpn-router`, loads on the first Tab after the command. For root, who can read them, the
+completions also offer the users and groups of the directory, the settings and site-to-site
+connections of `vpn-router.json` with `true` and `false` for a boolean setting, and the issuing CAs and
+certificate templates; for other users only the fixed words.
+
+```sh
+sudo -i
+vpn-config-edit set oidc.<Tab>
+vpn-router-user passwd <Tab>
+```
+
 ## Applying changes
 
 ```sh