Added reproducible ZIP example

[guss77]

Fixes issue #364

Also see here for a discussion: https://content.pivotal.io/blog/barriers-to-deterministic-reproducible-zip-files

[coveralls]

Coverage decreased (-3.9%) to 92.658% when pulling 5bf9a9c on guss77:patch-1 into 8887b70 on rubyzip:master.

[tilsammans]

@simonoff any chance this might get merged?

[GrimmDaddy]

zip -X -r foo1.zip foo; sleep 5; touch foo/*; zip -X -r foo2.zip foo; cmp foo1.zip foo2.zip

[hainesr]

Thanks for the PR.
The default in Rubyzip is now (finally) to record the last modified time of a file as it's added to the archive, and then preserve that time when extracting. So I think this note is no longer needed. Please raise an issue if that's not the case - I am interested in making sure Rubyzip can be used for reproducibility purposes.