doc: add note about key derivation

adds a note to the crypto docs passing along
the advice that openssl gives about what
key derivation function they recommend.

PR-URL: https://github.com/joyent/node/pull/8580
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Merge remote-tracking branch 'joyent/v0.10' into v0.10
Merge remote-tracking branch 'upstream/v0.10' into v0.10