Update CHANGELOG for rescheduling

CHANGELOG.md

@@ -13,6 +13,7 @@ IMPROVEMENTS:
  * core: Node events are emitted for events such as node registration and
    heartbeating [[GH-3945](https://github.com/hashicorp/nomad/issues/3945)]
  * core: A set of features (Autopilot) has been added to allow for automatic operator-friendly management of Nomad servers. For more information about Autopilot, see the [Autopilot Guide](https://www.nomadproject.io/guides/cluster/autopilot.html). [[GH-3670](https://github.com/hashicorp/nomad/pull/3670)]
+ * core: Failed tasks are automatically rescheduled according to user specified criteria. For more information on configuration, see the [Reshedule Stanza](https://www.nomadproject.io/docs/job-specification/reschedule.html) [[GH-3981](https://github.com/hashicorp/nomad/issues/3981)]
  * core: Servers can now service client HTTP endpoints [[GH-3892](https://github.com/hashicorp/nomad/issues/3892)]
  * core: Servers can now retry connecting to Vault to verify tokens without requiring a SIGHUP to do so [[GH-3957](https://github.com/hashicorp/nomad/issues/3957)]
  * core: Updated yamux library to pick up memory and CPU performance improvements [[GH-3980](https://github.com/hashicorp/nomad/issues/3980)]