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>

LICENSE

@@ -0,0 +1,26 @@
+GNU GENERAL PUBLIC LICENSE
+Version 3, 29 June 2007
+
+Copyright (C) 2026 Grzegorz Luszczek
+
+Everyone is permitted to copy and distribute verbatim copies of this license
+document, but changing it is not allowed.
+
+                            PREAMBLE
+
+The GNU General Public License is a free, copyleft license for software and
+other kinds of works.
+
+When we speak of free software, we are referring to freedom, not price. Our
+General Public Licenses are designed to make sure that you have the freedom
+to distribute copies of free software (and charge for them if you wish), that
+you receive source code or can get it if you want it, that you can change the
+software or use pieces of it in new free programs, and that you know you can
+do these things.
+
+To protect your rights, we need to prevent others from denying you these
+rights or asking you to surrender the rights. Therefore, you have certain
+responsibilities if you distribute copies of the software, or if you modify
+it: responsibilities to respect the freedom of others.
+
+For the full license text, see: https://www.gnu.org/licenses/gpl-3.0.txt