Update README, add new figure

Author: krustowski

.github/img/r2-kernel-boot.png

@@ -5,7 +5,12 @@ A second iteration of the RoureXOS operating system, rewritten in Rust.
 + [Original RoureXOS (a blog post)](https://krusty.space/projects/rourexos/)
 + [rou2exOS Rusted Edition (a blog post)](https://blog.vxn.dev/rou2exos-rusted-edition)
 
-![rou2exOS startup](https://blog.vxn.dev/images/posts/rou2exos/cover.webp)
+The goal of this project is to make the rou2exOS kernel to follow the microkernel architecture. For purposes of the external program development, there are two key links to consider opening when thinking about extending the system:
+
++ [ABI specification document](/docs/ABI_OVERVIEW.md)
++ [Syscall client implementation examples](https://github.com/krustowski/rou2exOS-apps) (aka `rou2exOS` Apps)
+
+![rou2exOS startup](/.github/img/r2-kernel-boot.png)
 
 To run the OS, you can use the attached ISO image from any Release, and run it in QEMU emulator. The system was also tested on the x86_64 baremetal (booted from the USB flash disk).