This PR:

• updates the readme.txt and mailchimp.php header values to the WP.org standards
• replaces references of MailChimp to Mailchimp except where those references are explicitly functional code (to limit the additional class/function updates needed)
• creates a LICENSE.md file and sets the license in the plugin to GPL-2.0-or-later
• moves screenshots into a new .wordpress-org directory
• creates a README.md file from the readme.txt content

Relates to #5. Note that additional work is still needed in #5 so this won't fully close out that issue as additional tasks still need to be handled there.