Add Changelog.

[aaronweeden]

Description

This PR adds CHANGELOG.md.

Motivation and Context

The Changelog will be used to document changes made in subsequent versions.

Tests performed

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• CHANGELOG.md has been updated
• The milestone is set correctly on the pull request
• The appropriate labels have been added to the pull request
• The following command produces no errors (replace your own token from https://xdmod.access-ci.org): XDMOD_API_TOKEN=YOUR_TOKEN_HERE pytest -vvs -o log_cli=true tests/