doc: add TLS session resumption example

Using TLS session resumption correctly is not obvious. This added
example code should help new users understand how to use it correctly.

Related issue: https://github.com/nodejs/node/issues/3132
PR-URL: https://github.com/nodejs/node/pull/3147


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>