bumping version for the 1.6 release branch (elastic#920)

Cut ECS 1.5.0 changelog (elastic#773)

Cut 1.4.0 changelog (elastic#701)

Cut the 1.3.1 changelog (elastic#669)

Cut the 1.3 changelog (elastic#648)

Cut changelog for 1.2.0 (elastic#578)

Cut 1.1.0 changelog, add contributor guidelines to changelog.next (el…

…astic#527)

Finalize 1.0.1 changelog (elastic#472)

Create new unreleased section in the changelog for 1.0.0 (elastic#376)

Prepare 1.0.0-beta2 release (elastic#271)