Clearer guidance about adding new `latest` section to changelog

[nathanjmcdougall]

The PR template gives a checklist item for:

[ ] Add a summary of changes to `CHANGELOG.rst`.

And the contributing guide notes that we should

Add a note to CHANGELOG.rst about the changes.

But since there have been no contributions since the latest release (3.14), there is no latest section, and it's not clear that this needs to be created. I had to use git history to work it out. It would be good to mention/explain this in CONTRIBUTING.md I think.

[seddonym]

Good idea, thanks!

[seddonym]

Have changed this in #280, feel free to review!