Create a use case in the USE_CASES.md file for making new themes on templates

[mbernier]

Add a section to the USE_CASES file called "Making a new template theme"

This should explain how to take a current template and apply new themes to it, as well as adding the new folder. Please include the following:

• Make a copy of the paste-templates directory, named after your new theme

cp paste-templates my-new-theme-templates

• Apply your changes to the CSS of these templates
• Change the name "Paste" in all the templates to the name of your theme
• Make a PR to this repo with the new files