How and when to add a changelog entry should be documented in the tkldev docs

[JedMeister]

How and when to create a changelog entry and the associated process should be clearly documented in the tkldev docs.

FWIW I have spelled most of it out in a comment here: #897 (comment)

Also relevant to #728 & #275. Side note to #1151.

[JedMeister]

I just realised that this is essentially a duplicate of #1125 - closing this one in favour of the other.