Add more details about the rootfs introduction

[allencloud]

Currently, the readme contains little information. Wish to add more details about the rootfs introduction, including what, why, who, when and some essential others.

[allencloud]

@kakaZhou719 Could you add guidance of what is the relationship between sealer and rootfs? What is the way that sealer uses rootfs?

[kakaZhou719]

@kakaZhou719 Could you add guidance of what is the relationship between sealer and rootfs? What is the way that sealer uses rootfs?

this rootfs repo save the static file from ClusterImage like config file and some scripts which will be merged with binary file from OSS to a completely ClusterImage by auto-build.sh . @bxy4543 is working on this feature that user could triggered it from github action by issue comment .