Adding changelog for 1.5 to 1.5.1 release (#5533)
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

gsantovena/marathon,gsantovena/marathon,gsantovena/marathon,mesosphere/marathon,mesosphere/marathon,guenter/marathon,meln1k/marathon,janisz/marathon,mesosphere/marathon,janisz/marathon,mesosphere/marathon,gsantovena/marathon,meln1k/marathon,janisz/marathon,guenter/marathon,janisz/marathon,meln1k/marathon,guenter/marathon,gsantovena/marathon,meln1k/marathon,guenter/marathon,mesosphere/marathon,janisz/marathon,meln1k/marathon,guenter/marathon