Add support statement to README

[samdark]

Adding a support statement to README would make the project more attractive:

1. Which versions of PHP are supported, when new version is added.
2. Is it officially supported by Shopware AG? If so, worth mentioning it.
3. Worth linking to support statements of base images.
4. Worth adding a section "Why not official FrankenPHP image?".

[shyim]

1.) All PHP supported versions official.
2.) Nope. maybe in future idk.

I will add the stuff later right now at a conference.

My intention for this repository is more like a "backport" while waiting that this package gets accepted to Wolfi itself. Therefore the Packages are here should decline in future. For FrankenPHP I am not sure, I didn't get any concrete feedback back from Wolfi if they are interested or not, but maybe I need to push that topic further :)

[samdark]

Please do. Thanks. FrankenPHP is quite interesting, especially in worker mode.