Add more information to readme

README.md

@@ -5,9 +5,26 @@ It uses the
 [Briar REST API](https://code.briarproject.org/briar/briar/blob/master/briar-headless/README.md)
 and therefore requires Java.
 
+## Installation
+
+So far, there is no official installation method and
+you have to build it from source.
+
+## Developers
+
+The easiest and most convenient way is to build _briar-gtk_ using
+[Builder](https://wiki.gnome.org/Apps/Builder).
+In _Builder_, click "Clone Repository" at the bottom and
+enter the URL to this Git project.
+
 ## Design Goals
 
-* Intuitive and simple UI, adaptive to different screen sizes (desktop and mobile devices)
+* Intuitive UI that follows the Briar Android client
+* Main platform is GNU/Linux, but should also support Windows and macOS
+* Follows [GNOME Human Interface Guidelines](https://developer.gnome.org/hig/stable/)
+* Adaptive to different screen sizes (desktop and mobile devices)
+* Has [phone constraints](https://developer.puri.sm/Librem5/Apps/Guides/Design/Constraints.html) in mind
+* Strictly separates API wrapper from GTK stuff, making former a solid base for other (commandline) clients
 
 ## License