Add release notes for Sabre 0.6.0
# Release Notes
+## Changes in Sawtooth Sabre 0.6.0
+
+* Add `add_events` to Sabre TransactionContext which adds events to the
+ execution result for a transaction. This brings the Sabre API closer to the
+ Sawtooth TransactionContext API.
+* Improve error messages when there is an InvalidTransaction caused by a
+ transaction not being submitted by an authorized admin.
+* Update sawtooth-sdk dependency version to 0.5.
+* Update transact dependency version to 0.3.
+
## Changes in Sawtooth Sabre 0.5.2
* Add `Result` class to the AssemblyScript SDK for basic error handling
