Standardize an approach to documenting implementation/project plan changes

[zackkrida]

Often in the project lifecycle we uncover problems which require alterations to our original planning documents. We want to update the documents to reflect the actual work that needs to happen, but we also want to preserve the original approach and make others aware that the document has changed.

We should develop a standardized method for addressing these updates. I would propose we add a changelog section to the plans like so:

## Changelog

- [Date of changes](Link to changeset PR diff) - Summary of changes made to the plan

I am open to alternative suggestions! I'll mark this as blocked until at least one @WordPress/openverse-maintainers has reviewed the proposal.