docs: add release cadence document (#5633)

RELEASE_CADENCE.md

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+# Release Cadence
+
+## Overview
+
+Atlantis follows a **monthly release cadence** to provide regular, predictable updates while maintaining stability for users.
+
+## Release Schedule
+
+-  **Frequency**: Once per month
+-  **Timing**: First week OR last week of every month (but only once per month)
+-  **Release Day**: Typically Tuesday or Wednesday to allow for weekend buffer
+
+## Versioning
+
+Atlantis follows [Semantic Versioning](https://semver.org/) (SemVer):
+
+-  **Major releases** (x.0.0): Breaking changes
+-  **Minor releases** (0.x.0): New features, backward compatible
+-  **Patch releases** (0.0.x): Bug fixes and security patches
+
+## Release Branches
+
+-  **Main branch**: Contains the latest development work
+-  **Release branches**: Created for major/minor releases (e.g., `release-0.20`)
+-  **Hotfixes**: Applied to both main and relevant release branches
+
+## Communication
+
+-  **Release Announcements**: Posted on GitHub Releases and community channels
+-  **Breaking Changes**: Clearly documented in release notes and migration guides
+-  **Security Updates**: Immediately communicated through security advisories
+
+## Release Criteria
+
+A release is ready when:
+
+1. ✅ All tests pass
+2. ✅ Documentation is updated
+3. ✅ Release notes are current
+4. ✅ No known critical bugs
+5. ✅ Security scan passes
+6. ✅ Performance benchmarks are acceptable
+
+## Emergency Releases
+
+In case of critical security vulnerabilities or severe bugs:
+
+1. **Immediate Assessment**: Evaluate severity and impact
+2. **Hotfix Development**: Create targeted fix
+3. **Expedited Testing**: Focused testing on the fix
+4. **Emergency Release**: Release outside normal cadence if necessary
+
+## Contributing to Releases
+
+-  **Feature Requests**: Submit early in the month for consideration
+-  **Bug Reports**: Report immediately for faster resolution
+-  **Testing**: Help test release candidates
+-  **Documentation**: Contribute to release notes and migration guides
+
+For detailed information about contributing to Atlantis, see [CONTRIBUTING.md](./CONTRIBUTING.md).
+
+## Release History
+
+For detailed information about past releases, see:
+
+-  [GitHub Releases](https://github.com/runatlantis/atlantis/releases)
+
+---
+
+_This document is maintained by the Atlantis maintainers. For questions about the release process, please open an issue or contact the maintainers._