doc: add caveats of algs and key size in crypto
doc: add caveats of algs and key size in crypto

Add description of user responsibility in the choice of cypto
algorithms and its key length. Some of recommendations for the safer
use are also described.

PR-URL: https://github.com/nodejs/node/pull/3479
Reviewed-By: James M Snell <2d64a127ffc94dbe6295928a16bedf8b26814159@gmail.com>

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