Update CONTRIBUTING guidelines on CHANGES

[gsnedders]

[sedrubal]

Does this include to update readthedocs? Because readthedocs documentation is outdated...

[willkg]

This is in CONTRIBUTING.rst currently:

- **Add a changelog entry** at the top of ``CHANGES.rst`` following
  existing entries' styles.

@gsnedders Is there anything else to do in this issue?

[gsnedders]

IIRC the vague unwritten policy had been to have end-user significant changes in CHANGES rather than everything, though maybe it's just too hard to judge significance.

[willkg]

Ok. I think that's a maintainer-job and not something we want to push on contributors unless we can figure out a way to explain it such that they're likely to succeed.

Given that, I'm going to close this out.