doc: Describe FIPSDIR environment variable

As per the OpenSSL User Guide, it is possible to use the FIPSDIR
environment variable to specify a custom install path for the
validated cryptographic module.

PR-URL: https://github.com/nodejs/node/pull/3752


Reviewed-By: Michael Dawsson <michael_dawson@ca.ibm.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>
Reviewed-By: Fedor Indutny <fedor@indutny.com>