Adding changelog for 1.5 to 1.5.1 release (#5533)

* adding changelog for 1.5 to 1.5.1 release

* updated based on PR feedback

* updated based on PR feedback

* more feedback from tim included

* last update from tim

changelog.md

@@ -1,3 +1,31 @@
+## Changes from 1.5.0 to 1.5.1
+Bugfix release
+
+### Fixed issues
+- [MARATHON-7576](https://jira.mesosphere.com/browse/MARATHON-7576) Changed default of UnreachableEnabled to (0,0)
+- [D907](https://phabricator.mesosphere.com/D907) TaskLauncherActor doesn't wait for in-flight tasks on stop
+- [MARATHON-7765](https://jira.mesosphere.com/browse/MARATHON-7765) Fixes issue in which /v2/info endpoint always returned 1.5.0-snapshot1, regardless of the actual endpoint.
+- [PR 5421](https://github.com/mesosphere/marathon/pull/5421) Added SchedulerPlugin to enable the ability to customize the rejection of offers. (see below)
+- [MARATHON-2520](https://jira.mesosphere.com/browse/MARATHON-2520) Improved logging around migration
+- [D1044](https://phabricator.mesosphere.com/D1044) EventStream implementation moved to Akka eventStream
+- [MARATHON-7545](https://jira.mesosphere.com/browse/MARATHON-7545) Initialize RunSpecTaskProcessor and RunSpecValidator at startup to early detect misconfiguration.
+- [D974](https://phabricator.mesosphere.com/D974) Plugin configuration or initialization issues are made more obvious, potentially causing Marathon to not launch.
+- [MARATHON-7707](https://jira.mesosphere.com/browse/MARATHON-7707)  Resident tasks now have an up-to-date agentInfo (agentId) when they are re-launched, rather than preserving the agentInfo as received during initial launch.
+- [MARATHON-7724](https://jira.mesosphere.com/browse/MARATHON-7724) Better socket error handling leader proxy.
+-  [MARATHON-7711](https://jira.mesosphere.com/browse/MARATHON-7711), [MARATHON-7338](https://jira.mesosphere.com/browse/MARATHON-7338) Under certain circumstances, resident tasks wouldn't relaunch when resources were available, and reservations wouldn't be freed. In order to address this, Marathon no longer suppresses offers from Mesos.
+- [PR 5432](https://github.com/mesosphere/marathon/pull/5432) App and pod validation errors for missing network name.
+- [MARATHON-1703](https://jira.mesosphere.com/browse/MARATHON-1703) Fixed issue in which constraints would not be properly evaluated when launching multiple resident tasks at a time.
+
+### 1.5.1 New Behavior
+
+#### mesosphere.marathon.plugin.scheduler.SchedulerPlugin
+
+This plugin allows to reject offers. Possible use-cases are:
+  * Maintenance. Mark agent as going to maintenance and reject new offers from it.
+  * Analytics. If task fails, for example, 5 times for 5 minutes, we can assume that it will fail again and reject new offers for it.
+  * Binding to agents. For example, agents can be marked as included into primary or secondary group. Task can be marked with group name.  Plugin can schedule task deployment to primary agents. If all primary agents are busy, task can be scheduled to secondary agents
+
+
 ## Changes from 1.4.x to 1.5.0 (unreleased)
 
 ### Recommended Mesos version is 1.3.0