Enhance documentation with a security advice

Hi,

I did read [this](http://gavinmiller.io/2016/a-tale-of-security-gone-wrong/) blog post. I think this blog post explains very well why it is a bad idea to store information gathered by zxcvbn together with
the password. In my point of view even within log-files you should handle these values like passwords themselfes.

Maybe this way somebody else will not make the same mistake.

regards
dieter

No target documentation file, test, or entry point is named. Read the repository documentation to find the appropriate place for a security warning based on the linked blog post, and define completion as clearly advising users not to store zxcvbn-derived values with passwords or in log files.

Written by the indexing model from the issue text.