Properly describe code structure in the documentation

We need to clearly describe code structure of this repository in the README.md or ARCHITECTURE.md files

Something like:

```
├── bin ---> Contains ubuntu-core-initramfs to build initrd.img
├── debian ---> debian directory to build deb package
├── factory ---> main skeleton of the initrd cpio archive
├── features
├── postinst.d
├── snakeoil
├── tests
├── vendor
├── COPYING
├── crypttab
├── grub.cfg
├── HACKING.md
├── initramfs.debug
├── LICENSE
├── README.md
└── spread.yaml

```

- bin ---> Contains ubuntu-core-initramfs to build initrd.img
- debian ---> debian directory to build deb package . Especially see "rules" file to see how it builds systemd and builds other relevant dependencies
- factory ---> main skeleton of the initrd cpio archive