Refresh readme & main PHP file

[jeffpaul]

Cleanup and refresh readmes and main PHP file

• update readme.txt per https://developer.wordpress.org/plugins/wordpress-org/how-your-readme-txt-works/
• update mailchimp.php per https://developer.wordpress.org/plugins/plugin-basics/header-requirements/
• update README.md to mimic readme.txt and any other GitHub-specific info
• update copy in readme's per Mailchimp constraints

[jeffpaul]

Noting that we're blocked here until Mailchimp completes their review of the readme for any brand-specific updates.