[docs] [issue17006] Add advice on best practices for hashing secrets

[Ezio Melotti]

Changes by Ezio Melotti <ezio.melotti at gmail.com>:


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stage: needs patch -> patch review
versions: +Python 3.5 -Python 3.3

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17006>
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