Add release announcements link, consolidate Community section

Collapse duplicate Support section into Community. Add link to
release announcements signup at primeradiant.com/superpowers/.

README.md

@@ -185,10 +185,6 @@ MIT License - see LICENSE file for details
 
 Superpowers is built by [Jesse Vincent](https://blog.fsck.com) and the rest of the folks at [Prime Radiant](https://primeradiant.com).
 
-For community support, questions, and sharing what you're building with Superpowers, join us on [Discord](https://discord.gg/Jd8Vphy9jq).
-
-## Support
-
 - **Discord**: [Join us on Discord](https://discord.gg/Jd8Vphy9jq)
 - **Issues**: https://github.com/obra/superpowers/issues
-- **Marketplace**: https://github.com/obra/superpowers-marketplace
+- **Release announcements**: [Sign up](https://primeradiant.com/superpowers/) to get notified about new versions