docs(readme): add plugin install instructions

Document the recommended installation path via Claude Code plugin
marketplace:

  /plugin marketplace add https://github.com/Nikolay-Shirokov/cc-1c-skills
  /plugin install 1c-skills@cc-1c-skills

Plugin install becomes the recommended option; drop-in copy and
switch.py-based installs remain documented as alternatives for users
who prefer them or who target other AI platforms.

Closes #18.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -16,6 +16,13 @@
 └── ...
 ```
 
+Или установите как плагин Claude Code:
+
+```
+/plugin marketplace add https://github.com/Nikolay-Shirokov/cc-1c-skills
+/plugin install 1c-skills@cc-1c-skills
+```
+
 Или используйте скрипт установки:
 
 ```bash