Cleanup and refresh readmes and main PHP file - [x] update readme.txt per https://developer.wordpress.org/plugins/wordpress-org/how-your-readme-txt-works/ - [x] update mailchimp.php per https://developer.wordpress.org/plugins/plugin-basics/header-requirements/ - [x] update README.md to mimic readme.txt and any other GitHub-specific info - [x] update copy in readme's per Mailchimp constraints
Cleanup and refresh readmes and main PHP file