Change Type Documentation: addUniqueConstraint

[carolhunt]

We need our current documentation on addUniqueConstraint re-written following the new template below. We need more in-depth information and examples.

Here is a template of the workflow:
https://docs.google.com/document/d/10nBYiGIQysz9HbB5BfVmxb61_td-xnw4yY49F1McGk8/edit

Here is the current documentation that needs to be re-worked:
https://docs.liquibase.com/change-types/community/add-primary-key.html

Please utilize the template to aid in the flow of your work.

Your byline, photo and link to your preferred social page will be included once completed.