erofs-utils: update README

update README for the upcoming erofs-utils v1.5, such as:

 - Image extraction with fsck.erofs;

 - Container image use cases.

Link: https://lore.kernel.org/r/20220611113621.359723-1-xiang@kernel.org
Signed-off-by: Gao Xiang <xiang@kernel.org>
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