Remove email address from Documentation/filesystems/overlayfs.txt

I'm getting a surprising large number of questions about overlayfs sent
to me personally, rather than to a relevant mailing list.

So remove my email address from the documentation, and add a note
about looking in the MAINTAINERS file.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>