BUILDING.md: Add Fedora instructions
BUILDING.md: Add Fedora instructions

Signed-off-by: Rajiv Ranganath <4acda8d1ee7b6cc93e786e53408b868c7be3fe76@atihita.com>
Reviewed-by: Stefano Stabellini <59f4e925dd388ea3d527c142f1436b5b8fb1f1a4@kernel.org>

rkt/stage1-xen,rkt/stage1-xen,rkt/stage1-xen