maintenance: new MAINTENANCE.md file (must read)

Signed-off-by: Eduardo Silva <eduardo@chronosphere.io>

Author: edsiper

MAINTENANCE.md

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+# Fluent Bit Maintenance Policy
+
+This document outlines the maintenance strategy and version support for Fluent Bit.
+
+## Active Branches and Maintainers
+
+| Branch     | Version            | Status              | Maintainer                                                  | Notes                                                              |
+|------------|--------------------|---------------------|-------------------------------------------------------------|--------------------------------------------------------------------|
+| `master`   | v4.1 (development) | Active development  | [Eduardo Silva](https://github.com/edsiper)                 | All new features and bug fixes land here first                     |
+| `4.0`      | v4.0.x             | Maintenance only    | [Hiroshi Hatake (@cosmo0920)](https://github.com/cosmo0920) | Critical fixes and safe backports only. Maintained until **Dec 31, 2025** |
+
+---
+
+## v4.0 Maintenance Policy
+
+As of **July 2025**, active development has moved to Fluent Bit **v4.1** (tracked in the `master` branch).
+
+**v4.0** has entered **maintenance mode**, now maintained by [Hiroshi Hatake (@cosmo0920)](https://github.com/cosmo0920), a long-time Fluent Bit contributor and core developer.
+
+Maintenance for v4.0 will continue until **December 31, 2025**, which is three months after the official v4.1 release.
+
+### Accepted Changes for v4.0
+
+- ✅ Security patches
+- ✅ Critical bug fixes
+- ✅ Low-risk enhancements that unblock adoption (e.g., OpenTelemetry improvements, performance tuning)
+
+> ⚠️ All changes must first be merged into `master` before being cherry-picked into the `4.0` branch by the maintainer.
+
+v4.0 releases will continue on an as-needed basis depending on urgency and impact.
+
+---
+
+## How to Contribute to Maintained Versions
+
+If you're submitting a fix or feature relevant to v4.0:
+
+- Open your PR against the `master` branch
+- Add a note in the PR or issue: `Target: v4.0`
+- Tag [@cosmo0920](https://github.com/cosmo0920) to request backport consideration