docs(ruleset): add readme file

[shorodilov]

This pull request adds comprehensive documentation to the ruleset/README.md file, introducing detailed information about rulesets in GitHub repositories. The new documentation explains what rulesets are, who can use them, their advantages over branch protection rules, and how they interact with forks and enforcement statuses.

Key documentation additions:

Overview and Usage:

• Provides an introduction to rulesets, including their purpose, who can use them, and which GitHub plans support them.
• Explains the distinction between branch/tag rulesets and push rulesets, including how to target branches/tags using fnmatch syntax and how push rulesets apply to entire fork networks.

Configuration and Permissions:

• Details how to grant bypass permissions for rulesets to specific users, teams, or GitHub Apps, and describes organization-level rulesets for enterprise customers.

Advantages and Layering:

• Outlines the advantages of rulesets compared to traditional branch protection rules, such as rule layering, visibility, and additional metadata controls.
• Describes how rulesets and branch protection rules are aggregated, with the most restrictive rule taking precedence when conflicts arise.

References and Further Reading:

• Adds multiple reference links to GitHub documentation for further information on rulesets, permissions, and syntax