Documentation is available online, though it may not be quite up to date or match your version exactly.
You can generate API documentation by running
phpdoc in the top-level folder of this project, and documentation will be generated in this folder:
php ~/vendor/bin/phpdoc -d ~/dkim-php-mail-signature/ -t ~/dkim-php-mail-signature/docs/
You will need to have phpDocumentor installed.
If this project helps you reduce time to develop, you can give me a cup of coffee ☕️ :-)