DOC: Changlog does not explain the usage pattern

[CJ-Wright]

There is a use pattern to get Changelog to work properly.

All PRs must have an addition to the news folder as a new file (or some other user defined file). During the release process the news info get collated and moved into a changlog file.

More broadly we may want to capture some critical use patterns in its own rever docs file. These patterns are not formally part of the rever code base but are needed to make the most of the software.

[scopatz]

Thanks for reporting this @CJ-Wright

[CJ-Wright]

Closed by various PRs into the docs