Update 2.13 release notes
Update 2.13 release notes

Add information aboout the 'Reject Implicit Merges' setting.

Change-Id: I822a572e13953bb6c40ced39cf417098caee32a4

GerritCodeReview/gerrit,gerrit-review/gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,WANdisco/gerrit,WANdisco/gerrit,GerritCodeReview/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,gerrit-review/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,WANdisco/gerrit,gerrit-review/gerrit,gerrit-review/gerrit,qtproject/qtqa-gerrit,qtproject/qtqa-gerrit,GerritCodeReview/gerrit,WANdisco/gerrit,gerrit-review/gerrit