* plugin/sign: track zone file's mtime

Resign if the original zone's mtime is change in some way.

Closes #4407
Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin/sign/README.md
Co-authored-by: Chris O'Haver <cohaver@infoblox.com>
Co-authored-by: Yong Tang <yong.tang.github@outlook.com>
Co-authored-by: Chris O'Haver <cohaver@infoblox.com>