Add README, LICENSE (GPL-3), and update config dir to XDG path

Config moved to ~/.config/portainer-cli/config.yml following XDG convention.
README covers installation, configuration, all commands, container name
resolution logic, and debugging. Tool noted as AI-generated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/portainer_cli/config.rb

@@ -5,7 +5,7 @@ require 'fileutils'
 
 module PortainerCli
   class Config
-    CONFIG_DIR  = File.expand_path('~/.portainer-cli').freeze
+    CONFIG_DIR  = File.expand_path('~/.config/portainer-cli').freeze
     CONFIG_FILE = File.join(CONFIG_DIR, 'config.yml').freeze
 
     attr_accessor :url, :token, :api_key, :ssl_verify