Add section on using @plugins.apply_plugins decorator

[sweep-ai[bot]]

This PR adds a section to the README on using the @plugins.apply_plugins decorator. The section covers:

1. Importing the Plugins class and decorator
2. Defining an LLM function to decorate
3. What the decorator handles (prepending prompt, checking for API calls, calling plugins, summarizing calls)
4. Returning the original response if no API calls are detected

The goal of this PR is to make the usage of the decorator more clear for users.

Fixes #128.