Add custom review rules feature to AI Code Reviewer

[codeanu]

Custom Review Rules Feature

This PR adds the ability for users to specify custom review rules that will be included in the prompt sent to the OpenAI API. This allows users to tailor the AI's feedback to their project's specific needs.

Changes

1. Added a new custom_rules input parameter to the action.yml file
2. Modified the main.ts file to read and process custom rules
3. Enhanced the createPrompt function to incorporate custom rules into the prompt
4. Updated the README.md with documentation and examples

How it works

Users can now add their own custom rules by specifying them as comma-separated values in the workflow configuration:

custom_rules: "Check for proper error handling, Ensure functions have proper docstrings, Verify variable names follow team conventions"