Composer.json installation support

[svandragt]

When completed, it's possible to install htmxpress for composer.json WordPress projects, as follows:

composer config repositories.htmxpress vcs git@github.com:svandragt/htmxpress.git
composer require svandragt/htmxpress:dev-main  --no-update
composer update svandragt/htmxpress --prefer-source

• Update readme

[svandragt]

This works (probably since 23a7e8f) , just need to update the readme.