Add CHANGELOG.md to improve change tracking and transparency

[kushivaradaraj]

# Description : Solving issue #141
This pull request adds a CHANGELOG.md file to the OneBusAway Developer Documentation project. While the project currently has release notes in various locations, a dedicated changelog will provide several benefits:

1. Improved transparency: A changelog offers a clear, chronological record of all notable changes.
2. Better organization: Changes can be categorized for quick reference.
3. Enhanced communication: It helps communicate changes to users, contributors, and maintainers more effectively.
4. Easier maintenance: A dedicated changelog is typically easier to update and maintain.
5. Industry standard: Many open-source projects use a CHANGELOG.md file.
6. Versioning support: It clearly shows changes between different versions of the documentation.

Changes made

1. Created a new CHANGELOG.md file in the root directory.
2. Updated README.md to mention the new CHANGELOG.md file.
3. Updated /src/guides/outdated.md to include a link to the changelog.

How to test

1. Check that the CHANGELOG.md file is present in the root directory.
2. Verify that the README.md file includes a section about the changelog.
3. Confirm that /src/guides/outdated.md includes a link to the changelog.

Additional notes

• The CHANGELOG.md follows the Keep a Changelog format for consistency.
• Initial entries have been added based on the existing release notes.
• Future contributions should update the CHANGELOG.md file as part of their pull requests.

[CLAassistant]

All committers have signed the CLA.

[kushivaradaraj]

@aaronbrethorst
Respected project maintainer,

I've added a CHANGELOG.md file to the project to improve change tracking and transparency. The changes include:

1. Creating a new CHANGELOG.md file in the root directory.
2. Updating README.md to mention the new CHANGELOG.md file.
3. Updating /src/guides/outdated.md to include a link to the changelog.

Could someone please review the changes and provide feedback? Your input would be greatly appreciated.