Enhance fuse-overlayfs instructions.

The `fuse-overlayfs` package provided by Ubuntu up to 19.10,  is not
recent enough and causes errors on `buildah commit`, for instance.

Adjust the rootless tutorial to point this out and to provide more
detailed instructions on how to obtain `fuse-overlayfs` and configure it
for use by `libpod`.

Signed-off-by: Leonardo Rochael Almeida <leorochael@gmail.com>