doc: Adding best practises for crypto.pbkdf2
doc: Adding best practises for crypto.pbkdf2

Added some information around usages of how to use iterations, how to
choose decent salts and updating the example to have a significant
work factor and to use sha512.

PR-URL: https://github.com/nodejs/node/pull/3290
Reviewed-By: Fedor Indutny <e39d94bae70283f13d24c278e04952d6dad9384b@indutny.com>
Reviewed-By: Shigeki Ohtsu <92fb37b64464f5ee20f58e183b0504024ecd6585@iij.ad.jp>

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node