The newest osbuild is needed for support for aarch64. The aarch64 images
are using GPT which requires stable partuuid which was included in the
latest osbuild release (11). This will be used to produce stable
image-info tests.
This aligns the layout of `README.md` with the osbuild project. It now
has a clear structure with individual sections that we can continue to
improve on.
This tries to salvage most of the information that was previously in
README.md, but removes the bits and pieces that were either moved into
the osbuild-composer(7) man-page, or feels misplaced in a project
readme.
