staging: erofs: add document

This documents key feature, usage, and on-disk design of erofs.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Cc: <linux-fsdevel@vger.kernel.org>
Change-Id: I9b3b4f301e43d33b9c535acf1bafa2e3cf861a76
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>