Add a bin/changelog script and changelog documentation

cc @rymai @DouweM @smcgivern

~~Eventually we'll probably want to add a real option parser (likely the stdlib OptionParser) to the script, but this is probably good enough for now. I'll open a technical debt issue for that after merging.~~ Nevermind, it was easy so I did it in this one!

Thanks @rspeicher, I think we could be more explicit in the option names, and id -> merge_request but other than that, nothing major, I'm excited! :D

Added ~889916 label

Mentioned in issue #23568 (closed)

Resolved all discussions

Added 2 commits:

• cd6b76db - Update bin/changelog to use OptionParser
• 21daaf5b - Update documentation for bin/changelog

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Added 1 commit:

• bf9befec - Add fail_with method to bin/changelog

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Added 1 commit:

• bf9befec - Add fail_with method to bin/changelog

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Thanks @rspeicher! What do you think about adding very simple specs for this?

@rymai I want to, yeah.

@rymai It'll probably have to be in a separate MR. Opened https://gitlab.com/gitlab-org/gitlab-ce/issues/23978.

Reassigned to @rymai

Mentioned in issue #23978 (moved)

Added 1 commit:

• 8305234e - Add specs for ChangelogOptionParser in bin/changelog

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@rymai Added some basic specs for the option parser to this MR.

Added 1 commit:

• e2140466 - Minor bin/changelog improvements

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Made some changes to the docs and rebased to squash commits, still waiting for that to show up here.

Reassigned to @rspeicher

Marked this merge request as a Work In Progress

Added 10 commits:

• 1dc69be2...49077651 - 7 commits from branch master
• 03b932df - Add a bin/changelog script
• 23312b48 - Add changelog documentation
• 5acb3230 - Add specs for ChangelogOptionParser in bin/changelog

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@rymai Good to go!

Unmarked this merge request as a Work In Progress

Reassigned to @rymai

Status changed to merged

Mentioned in commit daca93c4

Since when are we adopting this method to add a CHANGELOG entry?

@alfredo1 As soon as this was merged.