Adding the release team as maintainers

[richardlau]

Refs: nodejs/Release#557 (comment)

Would anyone object to granting the release team (@nodejs/releasers) write access to this repository? Recent changes here (#45) and in changelog-maker have broken the release team's workflow and it would be useful if the team were able to merge in fixes such as #50.

[richardlau]

Given no objections and a 👍 I've added @nodejs/releasers to https://github.com/nodejs/branch-diff.