Update CHANGELOG.md

Author: sentrivana

CHANGELOG.md

@@ -2,7 +2,11 @@
 
 ## 3.0.0a2
 
-- No documented changes.
+We're excited to announce that version 3.0 of the Sentry Python SDK is now available. This release is the result of a long-term effort to use OpenTelemetry under the hood for tracing. This switch opens the door for us to leverage the full power of OpenTelemetry, so stay tuned for more integrations and features in future releases.
+
+Looking to upgrade from Sentry SDK 2.x to 3.x? See the [full list of changes](MIGRATION_GUIDE.md) for a comprehensive overview of what's new. Looking for a more digestible summary? See the [migration guide in the docs](https://docs.sentry.io/platforms/python/migration/2.x-to-3.x) with the most common migration patterns.
+
+⚠️ This is a pre-release. If you feel like taking it for a spin, we'd be grateful for your feedback. How was the migration? Is everything working as expected? Is *nothing* working as expected? Something in between? Please let us know [on GitHub](https://github.com/getsentry/sentry-python/discussions/3936) or [on Discord](https://discord.com/invite/Ww9hbqr).
 
 ## 2.30.0