docs: add ROADMAP.md with planned features and upcoming releases

ROADMAP.md

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+# SimpleLogin Roadmap
+
+This document outlines the planned features and improvements for SimpleLogin.
+For the most up-to-date status, check the [GitHub Issues](https://github.com/simple-login/app/issues) and [Discussions](https://github.com/simple-login/app/discussions).
+
+---
+
+## ✅ Recently Released
+
+- Proton integration for seamless login
+- Admin-disabled mailbox support
+- Improved alias management UI
+- API v3 for alias creation
+
+---
+
+## 🔄 In Progress
+
+- Improved self-hosting Docker Compose support
+- OpenSSL 3.x compatibility for DKIM key generation
+- Better troubleshooting documentation for self-hosted instances
+
+---
+
+## 📅 Planned
+
+### Short-term
+- Docker image versioning clarity (always-updated `latest` tag)
+- Improved onboarding guide for new self-hosters
+- Clearer upgrade documentation between versions
+
+### Medium-term
+- One-click unsubscribe support for Gmail (see #794)
+- Hidden/private aliases (see #799)
+- User-defined default domain for alias creation (see #1038)
+
+### Long-term
+- JMAP API support (see #948)
+- Bounce Address Tag Validation (BATV) support (see #1082)
+- Email subaddressing support (see #990)
+
+---
+
+## 💡 How to Suggest Features
+
+Have an idea? Please submit it to our [Feature Request Discussions](https://github.com/simple-login/app/discussions/categories/feature-request).
+
+Community votes help us prioritize what to build next.