Document current release, and clean up branches

[geek-merlin]

This library: Great work! The branch organization bit me though.

Everybody who does
composer require phpmailer/dkimvalidator
is stuck on the year-old 0.3 release. Workaround: composer require phpmailer/dkimvalidator:dev-dev@dev

Everybody who does git clone git@github.com:PHPMailer/DKIMValidator.git works on the year-old main branch. Workaround: git clone git@github.com:PHPMailer/DKIMValidator.git -b dev

Please clean this up to not bite everyone. I can help or if you give me temporary permissions, do it.

[geek-merlin]

NOPE, dev-dev release is brokenfor me, it's really dev like it announces.