Update CHANGELOG for recent changes
Update CHANGELOG for recent changes

Why is this change needed?
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This helps people know the significant changes when we release.

How does it address the issue?
------------------------------
Add CHANGELOG entries, including moving the PHP version requirement
from the README to the CHANGELOG.

Any links to any relevant tickets, articles, or other resources?
---------------------------------------------------------------
https://3.basecamp.com/3093825/buckets/29124557/todos/5284412189
https://3.basecamp.com/3093825/buckets/29124557/todos/5284421778

Any screenshots?
----------------

Did you complete all of the following?
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- Run test suite?
- Add new tests?
- Consider security implications and practices?

DocRaptor/docraptor-ruby,DocRaptor/docraptor-ruby