Point users to an up-to-date fork in the readme

[stackotter]

Hi Tomas,

I have created a fork of SwiftGtk that:

1. is compatible with the latest Swift version
2. has a neater repository structure
3. is safer (no use of unowned)
4. is actively maintained

Given that the last change to SwiftGtk was 5 years ago and you probably aren't interested in maintaining it anymore, would you be happy to prefix the readme with a link pointing to my fork as the newer version?

By the way, thanks for making this package, it's significantly nicer and easier to build compared to the other Swift Gtk packages I have found, great job!