Add swift-opa to the swift package index

[jcubit]

Hi folks!

I'm excited about this project! It would be great to add the package to the Swift Package Index so that it can easily be found by the Swift community. Adding a package to the index is quite simple and would provide useful metadata to help with adoption.

Thank you for open-sourcing this project! 🚀

[philipaconrad]

I am starting to look into this, so hopefully we can mark this issue resolved soon. 🙂

I'm aiming to bring up a couple more core features and provide some more extensive docs before promoting the package for the wider Swift community.