[Direct PR][release note]: Atomic Distributed Transaction (#17079)

Signed-off-by: Harshit Gangal <[email protected]>

changelog/21.0/21.0.0/summary.md

@@ -8,6 +8,7 @@
         - [Deletion of deprecated metrics](#metric-deletion)
         - [Deprecated Metrics](#deprecations-metrics)
     - **[Traffic Mirroring](#traffic-mirroring)**
+    - **[Atomic Distributed Transaction Support](#atomic-transaction)**
     - **[New VTGate Shutdown Behavior](#new-vtgate-shutdown-behavior)**
     - **[Tablet Throttler: Multi-Metric support](#tablet-throttler)**
     - **[Allow Cross Cell Promotion in PRS](#allow-cross-cell)**
@@ -79,6 +80,17 @@ $ vtctldclient --server :15999 MoveTables --target-keyspace customer --workflow
 
 Mirror rules can be inspected with `GetMirrorRules`.
 
+### <a id="atomic-transaction"/>Atomic Distributed Transaction Support</a>
+
+We have introduced atomic distributed transactions as an experimental feature.
+Users can now run multi-shard transactions with stronger guarantees. 
+Vitess now provides two modes of transactional guarantees for multi-shard transactions: Best Effort and Atomic. 
+These can be selected based on the user’s requirements and the trade-offs they are willing to make.
+
+Follow the documentation to enable [Atomic Distributed Transaction](https://vitess.io/docs/21.0/reference/features/distributed-transaction/)
+
+For more details on the implementation and trade-offs, please refer to the [RFC](https://github.com/vitessio/vitess/issues/16245)
+
 ### <a id="new-vtgate-shutdown-behavior"/>New VTGate Shutdown Behavior</a>
 
 We added a new option to VTGate to disallow new connections while VTGate is shutting down,