Add CHANGELOG.md file to the zebra git repo

[mpguerra]

Motivation

Add a CHANGELOG.md file to the repo before the list of tagged releases gets too long.

Solution

PR creates a CHANGELOG.md file with the contents of previously tagged release notes in reverse chronological order (newest releases are listed first)

It also updates the release checklist to include addition of the CHANGELOG.md and its necessary updates.

Closes #2106

Review

@ZcashFoundation/zebra-team

Reviewer Checklist

• CHANGELOG.md displays correctly
• New release checklist items make sense and are done in the correct order

Follow Up Work

Continuously updating CHANGELOG.md as we tag new releases.
Consider adding an "Unreleased" section to the changelog to document upcoming changes.

[mpguerra]

This is not urgent at all, it might be good to get this merged in the next couple of weeks before the next tagged release

[oxarbitrage]

Looks good.

[teor2345]

Consider adding an "Unreleased" section to the changelog to document upcoming changes.

We'll have to be careful adding an "Unreleased" section to CHANGELOG.md. If we update it in every PR, then there will be a lot of merge conflicts.

[teor2345]

This looks good, but let's make sure the updated changelog is included in the new version tag.

[mpguerra]

Consider adding an "Unreleased" section to the changelog to document upcoming changes.

We'll have to be careful adding an "Unreleased" section to CHANGELOG.md. If we update it in every PR, then there will be a lot of merge conflicts.

We don't need to think about this now but maybe we can add any "relevant" open PRs at the time of updating the CHANGELOG.

[teor2345]

This looks good, I'll leave it for @mpguerra to merge