Add release notes for 0.3.0 release
# Release Notes
+## Changes in Transact 0.3.0
+
+### Highlights
+
+* The `contract-archive` feature has been stabilized. This feature guards the
+ smart contract archive (scar) functionality.
+* The `transact::signing` module has been replaced by the `cylinder` library
+ crate.
+
+### libsplinter Updates
+
+* Update the `sawtooth-sdk` and `sawtooth-xo` dependencies to `0.4`.
+* Add a `clone_box` method to `ContextLifecycle` that enables the trait to be
+ clone-able.
+* Add the `ExecutionTaskSubmitter` for submitting tasks to the executor. This
+ struct is thread-safe and allows a single executor to be used across threads.
+* Update the `families::command::make_command_transaction` function to take a
+ signer for signing the transaction.
+* Refactor the `XoTransactionWorkload` and `XoBatchWorkload` to take optional
+ workload seeds and signers.
+
+#### Scheduler
+
+* Add the `SchedulerFactory` trait for creating generic schedulers, along with
+ implementations for `MultiScheduler` and `SerialScheduler`.
+* Require implementations of the `Scheduler` trait to have the `Send` marker
+ trait.
+* Change the `Scheduler::cancel` method to abort and return any in-progress
+ batches.
+* Change the `Scheduler::cancel` method to send a `None` execution result if the
+ scheduler is finished.
+* Update all schedulers to shut themselves down when they are finished and have
+ no more batches to execute.
+* Fix a bug in the serial scheduler where a `None` execution result was not sent
+ when the scheduler was finalized with a non-empty batch queue.
+
+### CI Changes
+
+* Enable nightly builds to catch any breaking changes in dependencies or
+ upstream tools.
+
## Changes in Transact 0.2.4
### Experimental Changes
