erofs-utils: document erofsfuse in README

We are about to release erofs-utils 1.2, add some words
to README about erofsfuse itself.

Link: https://lore.kernel.org/r/20201205154141.14588-1-hsiangkao@aol.com
Signed-off-by: Gao Xiang <hsiangkao@aol.com>
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