Add information about SWT experimental work to README

[vogella]

https://github.com/swt-initiative31 contains experimental work to re-implement SWT. We should add a link to it to the readme so that other can discover this.

Also it would be nice to have some "Getting started with the prototyp" documentation in https://github.com/swt-initiative31 and link it from here.

[HeikoKlare]

We now have some documentation for getting started with the prototypes in their readme documents. Active work happens on the following prototypes:

• Custom-drawn widgets with Ski(j)a: https://github.com/swt-initiative31/prototype-skija
• Cross-platform GTK: https://github.com/swt-initiative31/prototype-gtk

There is also a prototype based on Swing that can be tested, but we currently do not plan to proceed with that technology: https://github.com/swt-initiative31/prototype-swing

If you experience any difficulties with following the descriptions in the readme documents, please let us/me know, so that we can improve them (or just make a PR with the improvements on those repositories).

@vogella would you propose to add those links to the readme of this repository or somewhere else?

[vogella]

Yes, sounds like you information to add.